I haven't tried it, but I've been thinking about getting a USB microphone too. Remember that JACK can only handle one audio device, so it's probably not completely trivial to record from an USB microphone (which is a separate audio device) and for example output to another soundcard using JACK.
Which brings up something I have had to deal with a fair amount lately. Is there a good way to handle multiple cards in Jack?
I was thinking of looking at my alsa config to link the two cards there, but then if I did that would Jack recognize that setup? Or would it only have the option of one card or the other?
Seablade