Yep. I actually don't have Comcast...I had Insight, which was then bought out by Time Warner (service has improved dramatically since TW took over by the way). The modem that was originally provided me was garbage and didn't support any of the higher speeds. Although, when it was Insight, it didn't matter, because they neglected the network for years until TW took over (the fastest speed available was 10mbit until the TW takeover, it took them a year but it's 50mbit now).
I just went and bought a Motorola SB6141, not only do I save the $6/month, I have a much better device, so when the 50mbit service was made available all they had to do was upgrade my service.