I hope Valve releases map packs. The only thing that dissapointed me was the fact that it was slightly shorter than they said. While it was significantly longer than Portal 1, I remember them saying it being more like an HL2 game, and thats not true at all. HL2 had like 13 hours of playtime, according to that one site above.
I could see more room for playtime in the GLaDOS chambers in the earlier chapters. Maybe even a complete release of all the first chambers with the new elements/overgrown look. While that'd obviously be kinda a waste as far as playtime goes, it'd look lovey.