I have this bug too.

Torpedo48's bug report has been changed to https://bugs.maemo.org/show_bug.cgi?id=7932

Vote if you have this bug please.

Settings - Internet Connections
- Search Interval 5 min, applies to noth wlan and cellulae
- Connect Automatically: Any Connection

This keeps me connected throughout the day.

Those were the settings I had. This happened to me when tethering my computer. After some time, n900 disconnected from t-mobile internet. A message came up on n900 screen saying "cellular data not available". A red bar appeared across the white gprs internet symbol. After some time it disappeared. When I looked for t-moble in the available internet connections list, it was not there. I waited for more than 10 minutes once. It didn't reconnect. After that, I rebooted the phone whenever that happned, and it would inevitably happen after about 15 to 30 minutes of tethering to the pc. This only happened to me while ththering. Not while using the phone's internet.
I changed the APN setting from internet2.voicestream.com to epc.tmobile.com. Now I can tether without a problem. It does not disconnect. Will update if problem reappears.

Ok, I keep disconnecting and NOT reconnecting while using epc.tmobile.com as my APN too. So it sometimes keeps connected for a long time. But I can't figure out a pattern in when it disconnects and when it doesn't. I tried other APNs too. Here is a list of APNs I tried.
epc.tmobile.com
internet2.voicestream.com
internet3.voicestream.com
wap.voicestream.com
They all act the same way. Internet disconnects without warning, and doesn't connect back. So it seems to be a problem with the N900, not any of the APNs.
It happens when I'm using the phone's browser to surf the web too. Not just when it's tethered to the computer. And it seems to disconnect only when some data traffic is taking place.