Can I load and save my Minecraft maps to My Dropbox?

You would need soft links to do that.

Create a soft link from your minecraft savegame folder to inside the Dropbox folder.

However, it will be quite suboptimal. Minecraft stores save games in many, many small files and Dropbox does not support folder-wide rollbacks.

What I would use instead is a source control versioning system.

First time setup

1. Learn the basics of Mercurial
2. Get Mercurial for your operative system
   • If you use a modern operative system, use your distribution's package manager.
   • For other operative systems, download mercurial here.
3. Run hg init on your save folder
4. Run hg add; hg commit to save the state of your game.

Backing up

1. Play the game normally.
2. When you close the game, run hg commit again to save the state of your game. Give a meaningful commit message (e.g. "got my first iron pickaxe!")

This, except maybe for the commit message, should be easily automated with batch files. Here's a very simple bash script for Ubuntu:

java -jar minecraft.jar
cd ~/.minecraft/
hg commit -m `zenity --entry --title Minecraft --text Announce your deeds!`

Going "back in time"

1. Notice your save file is corrupted.
2. • If you did commit last time you played, run hg rollback to undo the last commit and restore the save game to your previous save state.
   • If you did not commit last time you played, run hg revert to restore the save games to your last commit.
3. Be glad you bothered and keep on playing.

Dropbox backup

While we are scripting, we might as well have a single file copy of your game files to be put on Dropbox.

java -jar minecraft.jar
cd ~/.minecraft/
hg commit -m `zenity --entry --title Minecraft --text Announce your deeds!`
zip -r ~/Dropbox/Minecraft/savefile.zip .

This makes for a much more Dropbox-friendly solution -- just be aware you are backing up the repository, not just the game files. If you want just the latter:

java -jar minecraft.jar
cd ~/.minecraft/
hg commit -m `zenity --entry --title Minecraft --text Announce your deeds!`
hg archive zip ~/Dropbox/Minecraft/savefile.zip

Remote hosting

If you want to keep game files up to date between different machines, it is trivial to do so - even without Dropbox. You can have a remote copy of your repository for free at these hosting solutions. Choose one that will not complain about your not using their service for a non-code project... Intuxication for example.

Be very careful, however -- you want to avoid conflicts. Merging will not work, so you will need to always push after you play and always pull before you play. This is trivial to do if you are using a script:

hg pull #get remote changes 
java -jar minecraft.jar
cd ~/.minecraft/
hg commit -m `zenity --entry --title Minecraft --text Announce your deeds!`
hg push #push to remote server

Specific setup details vary based on your hosting solution.

Here's how I did it on Windows:

1. Backup your saves
   (located in %AppData%\.minecraft\saves\)
2. Move your saves to a folder inside your Dropbox or Live Mesh or whatever file syncing service
   (eg: My Dropbox\Minecraft\saves\)
3. Create a junction that points to your sync'd saves:
   mklink /J "%AppData%\.minecraft\saves" "Your Dropbox Folder\minecraft\saves"

More info on NTFS Junctions, Dropbox (2GB, cross-platform, referral link), Live Mesh (5GB, Windows/Mac only).

Update: Here's a handy batch file to backup your save games in a zip file before launching minecraft:

@echo off
echo "Backing up save files..."
"Your Path To 7zip\7-Zip\7z.exe" a -r -mx7 -tzip "Your Dropbox Folder\minecraft\saves.zip" "%AppData%\.minecraft\saves"
minecraft.exe

On the Mac, your worlds are stored in ~/Library/Application Support/minecraft/saves/. You can create an symbolic link at that location to wherever you'd like, Dropbox or otherwise, and Minecraft will follow that symlink to a different location.

Have you tried Sugarsync? It will allow you to sync specific folders on your computer as opposed to having to link everything through the dropbox folder.