From the bottom up...
in the you BIS site - go to profile and change your "sent from" address to be your Comcast address
under options you can also set up your "reply to" addres to be the same so that email always goes to your Comcast account
for your first question..
are you leaving a copy of the message on the server? by that i mean do you use a mail client like Outlook to view your email? if you don't leave a copy on the server there will be nothing there for the BlackBerry to poll when it connects.
also how long did you wait to get emails? polling occurs every 15 minutes - not right away; and i beleive it can take at least 20 mins from when you originally add your account (that's been my experience).
if all this was done (and you want to go back to POP access - and not forwarding your email) enter in your Comcast addres but wrong password - this will get you to a screen where you need to put in more settings like your incoming mail server. I don't know if comcast changed their server's recently but it's possible that the BlackBerry is using the wrong one (some ISP's have different incoming mail servers depending on where in the country you are). if you go this route just make sure to remove the forwarding rule - other wise you'll get duplicate messages on your BlackBerry
hope this helps