#1: Of course you could log into Comcast web-based accnt and delete.
A niffty alternative (if supported by Comcast in your area) is to setup IMAP interface with your account (supported by your Treo). With that setup, your Treo folders will sync with your Comcast folders .... Treo deleted messages are removed from your Comcast Inbox (and show up in a Deleted folder). The only issue I have is even when I permanently delete from Treo Deleted Items, when sync-d with client (non-Comcast in my case), the emails will go to a Trash Folder ... the only way I can delte them from the client is from web-login.
I like this alternative mainly because it also saves my Sent emails (syncs with) on the web server. Later I can move them from Sent folder to the Inbox folder from the web client and ultimately retrieve all my working history in Outlook then next time it checks for email.