We have a couple locations that require 24/7 access to Internet and our current provider (AT&T) while mostly good is not always up. My concern would be if I go with another provider (for example Comcast) I'm going to be subject to the same down time if it's in the "last mile". I for the most part don't know where the failure points are on the ISP side but I would imagine the large majority are within the last mile. I'd looked at Mifi or similar solution but have concerns about bandwidth caps and overall speed. Any suggestions would be appreciated.
If you really care about reliability/availabilty stop using broadband and get a dedicated non-DSL link (or two, fro different suppliers).
If you're using T1's or similar then almost certainly two suppliers won't share the last mile (it was that concern that made me assume you were talking DSL sorry) - basically each supplier will cable directly to you from separate sites - that should be all you really need.
Have you spoken to AT&T about their service quality?