On a program such as Microsoft Outlook, the program will download my e-mails out of my comcast.net account to my PC making them accessible only through Outlook.
On a BB, will the phone download the messages off the Comcast server or will I be able to read my e-mais on my phone, and still have them on my comcast.net so I can still access them from my PC?
There is a setting in your POP mail options to specify how long messages are left on the server after Outlook downloads them. Set that to 1 day. That will allow you to read the messages on Outlook, and still leave them on the server long enough for the BlackBerry to also download them. I know that's not the answer to the question as you phrased it, but I believe it will solve your problem.
Posted from my CrackBerry at wapforums.crackberry.com