Two important things will dictate if an arbitrary MacBook Pro will support two monitors:
- Does your MBP have Intel integrated graphics or one of the nicer discrete graphics chips?
- Does it have one or two Thunderbolt ports?
If you have Intel's graphics only (the Intel HD Graphics), as found in the 13" MacBook Pro (and the MacBook Air and some Mac minis), you can attach one monitor to the Thunderbolt/mini DisplayPort port.
If you have discrete graphics in addition to the Intel integrated graphics (as found on the 15" and 17" MacBook Pro models), you can attach multiple monitors even if you have only one Thunderbolt port. Remember, though, that all items on the Thunderbolt chain must be Thunderbolt devices until you get to the last item, which can be mini DisplayPort (this is because TB devices have a daisy-chain port; DisplayPort devices are not required to). All adapters for other display plugs that are currently (August 2011) sold are mini DisplayPort, not Thunderbolt, so they'll end the chain. Ergo, the only way to do multiple displays is to have Apple's Thunderbolt display (which has the TB daisy-chain port) and then have an adapter (or just another display) on that. When manufacturers start to make Thunderbolt display adapters, then you can get two of them and hook up whatever displays you want.
Additionally, two Thunderbolt ports allows you to use two Thunderbolt to DVI/DP/whatever adapters rather than buying a true Thunderbolt display that will allow a second non-Thunderbolt graphics adapter to be connected down the chain.
@EmmEff's answer is correct insofar as USB to DVI adapters and their performance issues and the Dual-Link DVI stuff.