How does Deus Ex: Human Revolution tie in to Deus Ex?

The "Missing Link" DLC has been announced (for October), but this will apparently be set in the time between Jenson leaving Hengsha and arriving in Singapore so will not carry the story forward past the end of the game, but it may well provide more exposition on how the games are linked.

However, they have already made some fairly strong links between DE:HR and the original. Specifically, the little hidden element from the credits (that you mention) infers that Megan Reed is working with the Bob Page (and therefore the Illuminati), so is presumably using Adam's DNA to help them develop the nano-augmentations that appear in Deus Ex and the "nanite-virus chimera" mentioned may presumably result in the Grey Death.

There are also other weaker links throughout the game, for example when Jenson detonates the bomb in Hengsha shipyard he inadvertently allows Tracer Tong (a major character in Deus Ex) to escape the island. There are others too, I'm sure, but I won't try to list them all here.

A few of the Wikia pages provide more in-depth information, here's a few that are the most pertinent:

Deus Ex: Human Revolution, Plot
Adam Jenson, 2027
JC Denton, Early Life
Nano-augmentation, The beginnings

No sequel to Deus Ex: Human Revolution has been announced. However, since it has done well so far I wouldn't be surprised if Eidos Montreal was starting work on one soon.

I took the bit after the credits to just be a quick little tie in to the original Deus Ex. It also fits in well with the intro movie.

It doesn't really tie in that much.

DX1 has something called killswitches which, naturally kill their augmented subjects when switched on. This is what megan is making.

The illuminati are mentioned, but it's not really explored. Same with FEMA.

Bob Page, the antagonist in DX1, is introduced briefly at the beginning and the end. Manderly's name is mentioned...

Basically the two games have little to do with eachother. The developers just made sure to mention just enough from the original to justify using "Deus Ex" in Human Revolution