what version of iOS you using?
It's sitting on "Loading..." on both my iPhone 4 running 4.0 and my iPad running 3.2. I'd guess it's a possible issue on the servers.
Has it worked for you before?
Mine did the same thing yesterday when I was messing around with 4.2.1 and the patched version of Cydia. I just assumed it was because I had patched it. Maybe not.For months now, yes.
Edit: rebooted iPad; hung on Loading for four minutes this time, but Cydia finally came up. Will try same with phone.