i didn't mean to suggest that they speak english, but language shouldn't be too much of a problem as long as you remain vigilent. here's the standard drill:

-have your address written in chinese (often a hotel business card)
-tell the driver you need a receipt (fapiao)
-know where you're going (by referencing your map)
-if they try to rip you off, give them what you think is fair and walk

perhaps the most important thing to remember is that there's no reason to fear beijing cabbies. there's an implicit understanding that they will get the third degree if they're caught trying to swindle a foreigner so they generally backed down when pushed.

fwiw these observations are based on cumulative knowledge gleaned from in upwards of 2,000 cab rides in china.

btw i also recommend dunhuang.