CAUTION: Rooting the new MyTouch3G Fender LE or MyTouch 1.2 (phones with a 3.5mm headphone jack on top) requires a different process. Be careful not to mistake flashing instructions for regular 32As which do not take into account this new hardware (these devices appear to be hybrid 32A/32B boards, thus ordinary methods will result in the touchscreen not working). It is possible to get root, but there are no custom ROMs as of February 2010. Two threads at XDA here and here explain the situation further. This is an evolving issue; please be patient with developers.
For standard 32As, 'rooting' usually doesn't rely on any exploit per se, because most 32A Magics are shipped with a fastboot compatible SPL installed. If you see the "3 skateboarding Androids" screen when you boot to fastboot (Volume Down + Power), you should be able to do the following: