Mojang has released the 1.7.1 pre-release and there has been 3 disturbing tweets from cpw.
@KrypticDot @TheKevin8082 complete world save rewrite, complete block and item identification rewrite.
EwyBoy @SeargeDP low to medium. Depends on how bad you've been
(In response to a question on how well an FML mod will survive 1.7)
@KrypticDot @TheKevin8082 we estimate about 20% of 1.6 is still gonna be recognizable in 1.7.
For those of you not in the know about the snapshots, these are the major changes in the snapshots:
 - More worldgen
 - Rewriting of a few thousand lines of code
 - Rewriting the sound engine
 - Resource pack redevelopmemt
 - Achievements & statistics overhaul
 - New color pallete for maps
 - Block IDs are deprecated (still usable, but will be removed in a future update)
 - Command changes
Well I guess we can forget about modded 1.7 until Xmas.