I set my gmail accounts up as imap on my iPad and all my folders show up.
You're using imap and not pop, correct?
Go to your gmail settings at gmail.com and make sure imap is enabled. It's NOT enabled by default.
The incoming server in iPad's mail program settings should be "imap.comcast.com" for example instead of "pop.comcast.com".
Ah, that is very possible that Comcast doesn't support imap. Quite a few ISP's do not. In which case you won't be able to specify server side folders that will show up on your iPad. If Apple would ever allow 3rd party email programs the problem would be resolved. I can't see control freaks such as Apple allowing that though.