I, too, have seen various ISP's offering T1 connections. One thing you must
remember is that you need to get the digital connection from the customer
site to your...this is not under your control. This is the local telco, and
they are going to charge the customer a point-to-point T1 installation and
recurring monthly charge. What you see advertised is, in my opinion, the
"port charge" which would be piled on top of the bill from the telco. Look
at it as a "regular" dial-up line. They pay for the second phone line to
the telephone company, and then they pay for access to you.
If you were to offer T1 connections, you would need a method for them to
come in and access your bandwidth. Just like you've got to connect to your
upstream, you have a DSU/CSU and a router. If you were to start offering
you and the customer would need DSU/CSU's, you'd probably have to upgrade
your router to a Cisco 3640 or the like (something that can handle multiple
serial interfaces) and your customer would need a router.
I'm sure there's money to be made at it, but you would need to watch your
bandwidth consumptions. You could "resell" the bandwidth in a similar
method that you sell more than one account per modem, but that would get
kinda hairy.
I hope this helps...
/ric