Random iOS Mail App Freezes? I've found myself depending more and more on the Mail app's ability to pull down mail from the servers immediately upon their arrival, and for the most part it works flawlessly.

Every now and then, however, the app will freeze and I won't know about it until I feel I haven't received new mail in a sufficient amount of time. Then re-launching the app, I find it's frozen. After a few seconds, it force closes, and I can re-open it.

Re-opening it after a freeze shows me several e-mails that I'd received and interacted with at the approximate time of its last freeze, even if they're not present in Gmail anymore.

Running iOS 5.0.1 (jailbroken), but it did it just as much back when I ran stock iOS.

Any idea what causes this?
Mine did this a couple times - I rebooted it (and left it off for about 15 minutes, although this doesn't make a difference in the real world I don't think) and the issue went away - although I presume you've tried this.

I'd try properly restoring the software back to non-jailbroken state (so you have fresh software entirely) and then (if necessary) re-jailbreaking it. Otherwise, depending on your model, restore back to vanilla setup and take it into the Apple store.