The problem is that there's no "dedicated capacity" on the Internet.
If I wander out to $HIGH_QUALITY_ISP_OF_THE_DAY and purchase a T1, yes,
the point-to-point circuit between me and $HQIOTD's POP is dedicated
capacity. But the "dedicated" probably ends there.
It's possible, but unlikely, that a certain amount of the bandwidth
between $HQIOTD's POP and $HQIOTD's peering hub is dedicated bandwidth
set aside for my T1. Yet it would be fairly common for such a circuit
to be a DS3, with maybe 50-100 T1 customers sitting on the far side.
That probably isn't dedicated bandwidth, then.
Now, as you get off $HQIOTD's network, the problem escalates. Does that
T1 guarantee, for example, 1.5 megabits of bandwidth out to my GPRS modem
on my laptop? 
If you want a specific capacity 24x7x365 *FROM* one point *TO* another,
then by all means, get a T1. It is absolutely correct that this is
dedicated capacity.
However, I am tired of hearing that "if you want guaranteed Internet, get
a T1." It doesn't map that way. Internet is inherently shared, for any
meaningful definition of Internet. Buying a T1 doesn't guarantee you 1.5
megabits of capacity to the destination of your choice. It may buy you a
circuit where your ISP cares more about you, and works harder to make sure
you get your bandwidth, but that's not due to any technical requirement.
The real issue isn't transmission technology, but oversubscription, and
what sort of service you've promised to users. Users who buy a "T1" will
expect to be able to use most or all of it from time to time, and at the
prices they usually pay, the ISP will usually sit up and pay attention if
it's a problem. You can buy a business class DSL where you get that same
level of service - and the same sort of bandwidth "commitment".
I can absolutely, positively build you an ISP that guarantees full
bandwidth for every user from the CPE to the ASN border. (I seriously
doubt that it will be economically feasible, currently, but anyways..)
The problem is that even with that, the instant data leaves the ISP
network, you can no longer guarantee capacity.
... JG