Apologies for bumping an old thread but I'll do it anyway just in case it helps even one person looking for the answers I was.
Recording video using an external microphone is possible, using one that's plugged into the 3.5mm socket. It appears, however, that you need to be running a ROM that's based on 2.3.5 or higher.
Another thread (which I now can't find) said that you might need a 'Y-splitter' (to split the phone's 3.5mm socket to separate headset and microphone connectors), but these cost very little.
HTC Touch2 - Stock
Motorola Milestone - CM7
Samsung Galaxy S2 - CheckROM Revo/CM9a