So this tip probably applies to about 3 people out there besides me, but the road to a solution was about three days of pain, so I feel compelled to post something.
I've been a happy iPhone user since the beginning of 2009 when my manager at work sold me his 2G. A buddy of mine showed me the ropes of jailbreaking and unlocking the iPhone and before I knew it I was running iPhone OS 2.2.1 via QuickPwn on the T-Mobile network. Better yet, I didn't have to have a data plan (yes, I'm cheap), like you do with AT&T. I just hopped on an available WiFi network whenever I needed to use data - life was good.
Until I decided it would be a fun idea to upgrade my iPhone to the new 3.0 OS.
The actual install itself (I used redsn0w) was quite painless and relatively easy. After I got all my apps re-installed and my settings back to what I had before, I had a sweet phone that couldn't receive incoming calls or text messages. After several re-installs of the OS, attempting to use trial and error to narrow down the source of the problem, I finally stumbled upon the answer. And, yes, I tried googling the heck out of the internet and could not find another poor soul out there who was in my same predicament (hence the first sentence of this post).
The source of the problem was that I had Push turned on (which is the default when you install OS 3.0). Push is a technology that allows applications like email and calendar to receive updates from the server without having to continuously poll the sever. Polling like this is also called Pull, the opposite of Push. Push requires a data plan to work. I don't have a data plan with T-Mobile. I guess Push doesn't behave well without one. Kinda makes sense since I'm sure the iPhone developers didn't really think about this scenario since I think you're required to have a data plan with your iPhone if you're with AT&T.
Anyway, to turn Push off, here's where you go in the iPhone settings:
Settings > Mail, Contacts, Calendars > Fetch New Data > Push > Off
After I did that, everything works! And I have to say the 3.0 version of the iPhone OS is a nice upgrade, even on a 2G.