I confirmed this after reading a comment from TUAW reader BigB who mentioned it in another post. Apparently this is due to the security certificates that Google uses with its Exchange servers. The servers can force security options on compatible devices, which your iPhone becomes once it is running iOS 4.
To be clear: if you have a regular Gmail account and set it up using Exchange, this auto-lock requirement will not be triggered. But if you use Google Apps for your personal domain, it may.
Also: Google Apps users have also reported problems setting up their iOS 4 devices. Mine kept telling me that the server (m.google.com) was unable to be verified even if you followed the instructions.
Since I knew my information was correct, I went ahead and saved it. Email began syncing not long afterwards, but apparently the issue is still cropping up intermittently for some users.