Yep, the major VoIP providers are doing that now. (Vonage, Time Warner, etc.) My cable connection is highly reliable - only down once or twice a year over the past 5 years - but it's still not as reliable as my landline. I can't see ever losing the landline altogether.
We got rid of our land line. If we lose our net connection, we have the cell phone. If the cell phone goes down, we always have the neighbors:)