What does the StarCraft 2 client download when entering the league queue?

When I go to join a multiplayer game in StarCraft 2, sometimes the game client says it is downloading a few items before it enters the queue. This notice appears at the top center of the screen, and is soon replaced by the the "looking for players" message and then the "players found" message.

What is it downloading? Presumably I already have all the game content, and also presumably it can't fetch data on my opponents (such as their avatars) until it has decided who to match me with, which can be minutes later. Blizzard isn't issuing micro-patches this way, are they?

Edit: After the recent patch, these downloads took even more time. I'd love to know what's being downloaded, so I'm adding a bounty.

I noticed earlier today that as of 1.1, the Blizzard maps were updated from v0.1 to v1.0. (Agria Valley is currently at v1.2.) Also, in the SC2 Editor, some Blizzard maps are showing up as not downloaded:

I haven't played 3v3 since pre-1.1, so all the updated 3v3 maps are still marked as Not Downloaded.

This supports Mag Roader's theory that the downloads consist of map data.

So, here's what I think it is: When you join the matchmaking queue, the game asks Battle.net for a list of maps you might about to be playing on. If you have the latest versions of all the maps, this will probably only take a second.

If any maps are out of date, however, the game will update them. This will take a few seconds for each map since they're around a megabyte each.

This system would let Blizzard publish updated versions of maps, or add new ones to matchmaking, between actual patches.

(Probably) The maps! You don't start with them all (or any?) and must download the appropriate player # maps before you can play.

Presumably, yes, this does mean Blizzard can do micro patches on the maps themselves without needing to change the game client.

Edit: As per comments, it could also be doing a small check to ensure your maps are in fact up-to-date and not hacked. This wouldn't necessarily require downloading the full maps, but would require downloading something (albeit small) for verification. But it's still likely to be map related.

Edit: Also, yes, my answer is speculative. The people who know the answer for sure are people who have looked at the game code. If such people reply with a better answer, please upvote that! :D

Edit: The other day I moused over the downloads as they were coming in. There was a tooltip and, indeed, it claimed to be downloading maps.

No maps, no avatars, no updates, but:

Every time you search breaking news is requested from customer.teliacarrier.com.

Confirmed with Process Monitor, it first makes a request to a subdomain of that URL and then saves a file in

C:\ProgramData\Blizzard Entertainment\Battle.net\Cache\Download

that is called breakingNews.txt which is empty but allows them to interrupt the league queue if needed.

You can confirm this yourself looking at the modified times of the files, the only other things you might encounter that have recently changed (but not when entering the league) are the news images...

I guess the first time after you start the game your league settings are updated to match the league you are allowed to play in; so for example if you play in Practice you will have the slow settings and if you advance to ranked leagues you will have faster settings.

Why: Because I guess this doesn't happen when you load SC2 itself so you can go to single player without waiting for your slow connection. The profile data probably contains less or different data so it isn't updated in that case either. And it should always be made up-to-date when you enter multi-player in case you have played SC2 before on another computer...

Files: Besides creating/accessing some cached .s2m* files, the only readable file I could find contains this:

<?xml version="1.0" encoding="utf-8"?>
<locale 
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:noNamespaceSchemaLocation="l10n_output.xsd"
  locale="enGB">

  <e id="1">1v1</e>
  <e id="2">2v2</e>
  <e id="3">3v3</e>
  <e id="4">4v4</e>
  <e id="22">2vA.I.</e>
  <e id="42">2vA.I.</e>
  <e id="52">2vA.I.</e>
  <e id="32">2vA.I.</e>
  <e id="62">2vA.I.</e>
  <e id="23">3vA.I.</e>
  <e id="43">3vA.I.</e>
  <e id="53">3vA.I.</e>
  <e id="33">3vA.I.</e>
  <e id="63">3vA.I.</e>
  <e id="5">FFA</e>
  <e id="11">1v1</e>
  <e id="12">2v2</e>
  <e id="13">3v3</e>
  <e id="14">4v4</e>
  <e id="15">FFA</e>
  <e id="1001">1v1</e>
  <e id="1002">2v2</e>
  <e id="1003">3v3</e>
  <e id="1004">4v4</e>
  <e id="1022">2vA.I.</e>
  <e id="1042">2vA.I.</e>
  <e id="1052">2vA.I.</e>
  <e id="1032">2vA.I.</e>
  <e id="1062">2vA.I.</e>
  <e id="1023">3vA.I.</e>
  <e id="1043">3vA.I.</e>
  <e id="1053">3vA.I.</e>
  <e id="1033">3vA.I.</e>
  <e id="1063">3vA.I.</e>
  <e id="1005">FFA</e>
  <e id="1011">1v1</e>
  <e id="1012">2v2</e>
  <e id="1013">3v3</e>
  <e id="1014">4v4</e>
  <e id="1015">FFA</e>
</locale>

I think blizzard check that maps are recent ones. E.g. if they make small bugfix to map, then they just update it on their server, and system distribute it across players. As Mag already mentioned it most probably hashes, that your game send to server, and server decide do you need to update map or not.

I think it is pretty stupid that this checks done for every match, as these updates quite rare. But maybe it is done against hackers.

PS: it is also just an assumption, so unless someone verify it with packet spoofer we can't be sure.