A certain percentage of your allocated bandwidth will be used for overhead and error correction. It's extremely rare to reach the full theoretical speed.
Cool, thanks for all the feedback!
The Speedtest.net servers can't keep up with my test :P
It shouldn't, unless you have less than a 100% clear signal. Your router should be connected to your modem with at least a 54Mb transfer rate, so it shouldn't slow down much.
Where you probably could have a speed problem is ping time, which is really only a concern if you're a competitive gamer, where ping time could be critical (the difference between a headshot and being shot).