Gavin has stated that he wishes to pull features in the version .5 release candidate this weeks. He further states maybe the version .6 release will have it as a killer feature.

Maybe the big feature for version 0.6 can be fast initial download (I'm thinking the best thing to do for brand-new, starting-with-an-empty-wallet installations is to download only block headers, function as a 'lightweight' client, and 'backfill' full blocks until you're caught up with the full chain-- then function as a 'full' client)

https://bitcointalk.org/index.php?topic=46493.msg553778#msg553778