Generally speaking, here's what I've found from much testing with Break Speed with amateur and pro players.
The typical amateur breaks at around 17MPH. A serious amateur that practices their breaks a bit will often hit around 21-22MPH. Breaking at 24MPH is pretty good. Getting past 24MPH requires that you study the mechanics of the break shot seriously and practice a lot in order to improve body position, speed and timing.
Pros can usually get in the high 20's, but rarely do so in tournament play because it is very difficult to maintain control with such power.
A pro that is simply going for a full-power break (ignoring control completely) will sometimes hit 30.
Real-world examples:
I instigated a bit of competative fun between Corey Deuel and John Schmidt at the 2010 US Open 10-ball Championships with Break Speed. John was hitting 24 (full-out) and Corey broke through the 30MPH barrier after a few tries (and a few miss-cues.)
Last night, when we were filming the 2010 US Open 10-Ball in Vegas, I was timing Roberto Gomez at 28.2MPH with considerable consistency (only deviating as much as 0.2MPH from break to break.) His next match, he was taking a bit of power off his break (presumably to get more control) and was breaking in the 27's. This is VERY rare for tournament play, and shows how great of a breaker he is.
Break Speed can capture breaks from recorded matches (this is a challenge for the software, so the commentator must not be talking over the break) - and I timed Bustamante at 26.2 with great consistency.
One of guys that works for OB Cues (Steve Owens, a local amateur) claims he can break 30MPH after he warms up. In fact, his typical break during a match is 28MPH, so I'm willing to bet he can. In fact, I think he can hit 32. I will get this on film and maybe include some instruction to put on our site. Steve's a great guy and has an absolutely incredible break!
I was able to get my break from 17MPH into the 24's within a week. I did this while I was writing Break Speed, and thought I might as well work on my break while testing the software.

I did this by using Break Speed to tell me what made a difference (even if only a 1/2 MPH improvement) and following the tips in this video:
http://www.youtube.com/watch?v=xW1tsONEI_U
That video is by far the best video I've seen on break instruction - it covers all the body mechanics and gives explanations for them. If you watch the power breakers, they all incorporate the things in this video.