If you read this and still decide that you want a headstart on 4.0, here are a couple points you ought to take note of:

1) Make sure you “Restore” to install (as opposed to “Update”). There seem to be more issues with the latter.

2) 4.0 updates your baseband, so while you technically can revert to OS 3.1.3, Apple doesn’t recommend it, and some users have confirmed this- there could be severe incompatibility issues.

3) Not every developer wants/needs/can afford a testing specific device, but in some cases it really does come in handy.

And finally, 4) If you don't have an iPhone 3GS or latest model iPod Touch, don't expect to use Multi-Tasking. This isn't a bug, the feature is unsupported on your hardware.