Sorry it's taken me so long to respond to this; it's been a crazy week.
I'm also using the Intel 3945 ABG wireless card, and it's a nightmare. Sometimes it connects just fine, sometimes it doesn't. Sometimes it keeps prompting for the wireless key over and over without actually connecting. More often than not, the connection just quietly dies in the middle of downloading a large file, particularly if I was getting a decent download speed.
There are a number of bugs
filed on this, so you're not alone. The kernel devs and the team at IntelLinuxWireless.org
* are working hard on this. It's just slow going.
The best work-around I can offer you is this: when your connection dies and/or you don't see your wireless AP listed, right-click the NetworkManager applet, uncheck "Enable Networking", count to 20, and re-enable networking through the applet. It takes anywhere from 10 seconds to a minute to get the wireless up and going again, but you'll be able to connect... at least, until the next time it flops out on you.
Keep your fingers crossed, and I'll be suffering right along side you.
*Unless you're really keen on recompiling your kernel yourself, I wouldn't advise downloading the packages from these sites yourself. One of the developers there also builds the Fedora kernels, so they keep the updates pretty well synced.