Wait is over guys. Blacksn0w RC2 unlock for iOS 4 baseband 05.11 is here. It’s a repackaged version baked by Steve of PushFix fame. So, there’s nothing new here. Steve has patched the original blacksn0w RC2 with the patch recently released by msft.guy. So, it avoids you the hassle to blend patch the blacksn0w RC2 by yourself, as you had to do with PwnageTool. It is, however, strictly for iPhone 3GS and is NOT the official release of Blacksn0w RC2. Here are the release notes from Steve:

Use on Modem Firmware 05.11.07 ONLY!
V1.2: Support for 4.0 GM. Backwards compatible to 3.1.2/3 as well. V1.1: MyWi tethering fix
Geohot’s unlock for 3.1.2/3.1.3/4.0GM on the iPhone 3GS (and probably 3G). Please donate at http://blackra1n.com/ if you enjoy.
With 3.1.3 & 4.0 GM patch (when needed) by msft.guy. Packaged by Pushfix.info
This is an unofficial release but Geohot HAS given me permission to redistribute blacksn0w. All known issues with WiFi loss and boot loops seem to be fixed.  If it doesn’t work for some reason, you should still get the SpringBoard. This allows you to uninstall if necessary. It is a little slower to boot because of the increased safety of the release. A 3GS boots to cell connection established in about 90 seconds in iOS 4 now (50% slower than 3.1.X).
Improvements may be made in the future.

As you’d imagine from notes, more work still needs to be done. You better wait for the official update of iOS 4 on June 21. However if you can’t wait, you can download iTunes iOS 4 torrent or from the developer center (recommended), if you have a developers account. After downloading, you should jailbreak iPhone 3GS with PwnageTool so that you preserve your baseband 05.11. Although you can jailbreak iOS 4 with redsn0w B5  but it will update your baseband to 05.13.