Here's my frame of mind at the moment.
Until 4E is released, keep on keepin' on with 3.5 conversions.
If we like 4E, we'll probably want to devote all our time to 4E conversions (think of all the monsters ripe for converting again), and the 3.5 will stick around as legacy on the CC (much like the 3E versions still are present post 3.5).
If we dislike 4E, we might keep plugging away with the CC 3.5 conversions until all is done or we burn out.
Based on the current trend of design philosophy, the words "faster play" and "less prep time", I'm guessing I'm more likely to fall in with the latter option.