Apple just seeded Beta 6 for iOS 5 to developer accounts. Beta 5 was released two weeks ago, and with the iPhone announcement due within a few weeks, iOS 5 should be just about done. Here's some of the polish added in Beta 6.

There are a lot of bug fixes in this release. iCloud is a lot easier to enable, and bugs involving settings not saving have been fixed. Other various bugs were addressed in Calendar, iMessage, Safari SSL certificates, Picture syncing, and Wi-Fi syncing for contacts, calendars and account settings. There were also developments, fixes and improvements to various dev tools.

This release is mostly house cleaning, and should leave iOS 5 in a mostly-maybe-entirely usable state. iPhone day draws near.
http://gizmodo.com/5832652/ios-5-beta-6-is-out-now