I've had a breakthrough on my suspend issues. It seems that the Dell Connection Manager Service was causing the suspend problem. As a reminder, I have the 5530 HSPA AT&T card. When you install Dell Control Point (DCP) and the associated drivers, you get Dell Connection Manager, which is supposed to help manage all of your network devices. Mostly it's redundant with the Network Center in Vista, but you do need it to connect to the Internet over the WWAN card.
As I've complained before, I don't like DCP. It's Dell QuickSet gone horribly wrong. So I've been actively trying to replace it. In my searching for something else to manage the WWAN card, I found an alternative utility called Dell Wireless Manager. It's bundled with the 5530 drivers, but if you poke around on Dell's FTP site, maybe you can find it for the other WWAN cards, too. It's a nice little app for enabling/disabling the card, connecting, looking at bandwidth usage (very important now that AT&T's data plan is 5GB / month), etc. It doesn't work right away when you install it, though, saying that it can't detect a WWAN card.
Quite by accident I got it to work by uninstalling "Dell WWAN ???" (can't remember the exact title) using CCleaner. I was trying to remove this utility since it wasn't working, but instead I uninstalled something from Connection Manager. Suddenly this nice little app found the my WWAN card and connected to AT&T. The Dell Connection Manager service stopped working, and Suspend has been flawless ever since. I guess I killed two birds with one stone.
Here are some tips if you go this route. In BIOS, set the Wireless Switch to not control the WWAN card. The Dell Wireless Manager turns the card on when you start the utility and off when you close it. Also remove "DellConnectionManager" from startup (via msconfig or WinPatrol). It won't start up correctly anymore, and you won't miss it anyway. BTW, the rest of DCP is still working. DCP only gives you an error when you click Connection Manager. Perhaps next I'll try getting QuickSet to work and then blow away DCP for good.