I’m not 100% sure it’s the root cause, but it seems like having the W3 Total Cache plugin enabled messes with upgrades to 3.3/3.3.1. At least, when I had it enabled upgrades would present a blank screen in the admin UI, and disabling it allowed them to proceed as normal. (On the whole I am happy with W3TC for performance reasons; this host is a fairly small Xen-hosted virtualized environment so it’s not the brawniest machine ever.)