A millibit is a useless unit. I guess this falls under the same rule for talking about momentary figures:
"How much is the house worth?"
"395"
The house obviously isn't $395, it's $395,000. It doesn't make sense for it to be anything else... just as it doesn't make sense for 54mbps to be anything other than 54Mbps. In context, you aren't going to confuse someone if you use mbps or mb instead of Mbps.
Anyway, the signal quickly degrades with distance so the actual throughput of the wireless network could fall well below the maximum of 54Mbps. If distance isn't an issue, I guess it's possible that the network has fallen back on wireless-b since most wireless-g routers are backwards compatible... could just be a configuration issue.
Are you just browsing when you notice the slow down or are you transferring files/some other heavy network usage?