I don't think we'll see an iLife 12. iLife 11 was unveiled in October of 2010, so if they do the same thing, we may see iLife 13 released this fall. Of course there was no iLife 10, they skipped a year and this may also be the case.
Nobody knows if Apple will release a major new version of their apps (iLife or iWork) as new apps on the App Store - requiring everyone to purchase them again or they will just continually improve upon the exiting apps (with a major but free update now and then). I suspect the latter but nobody's sure.
I think if you buy all three iLife apps on the App Store right now, you'll be good - just do it, they're great the way they are now for their purpose.