Someone slipped up earlier today and posted links to ESX 3.5 Update 1, ESX 3i Update 1 and VirtualCenter 2.5 Update 1 on the VMware downloads page (there was nothing actually available for download though). Presumably that person thought the world runs on California time or something, as the links disappeared a short time later.
Anyway, April 10th looks to be the release date for Update 1 so if you're planning anything significant in the near future, might be worth holding off to see exactly what is included in the update.
I just noticed that 3i has a name change to ESXi in that screenshot too... good to see the marketers getting their share of this release (assuming it's not a typo)!