When will the official client support partial blockchain downloads? Are there concrete plans for the official client to be able to download just a portion of the blockchain?

By concrete I mean what's the issue tracking this on github, is there a planned version for it?
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