I suppose it could be that my device is not functional in that input. I did get it used. But it definitely differs in function from the '1/8" stereo mic input' of my nVidia MCP61 that's part of the onboard soundcard of my desktop, or the ATI IXP mono input of my laptop. It even says it differs in the MobilePre manual. As far as I can tell it's a difference of a bias voltage(+5V). But I wont know if that's the case until I get the adapter to the adapter. BTW I have about $200 worth of various 1/8" bias voltage powered mics. So it's not really a matter of $89 for one mic. As I'll in essence be throwing away $200 in other mics if I don't go the adapter route. I suppose I could make my own adapter for much less.
Powering microphones
With that being said, I am looking at other "real" microphones in addition to existing equipment and the SM57 is NOT my first choice. I suppose that if I could get one used for $50 I probably would. But as far as XLR mics, I'd want something higher end. Or that at least specs to 20Hz - 20kHz. The Cascade FatHead II being rather nice. In the meantime I'm scoping out craigslist for other deals. I don't imagine that I could walk into radio shack and pull an SM57 off the shelf. Or maybe I could, I haven't really checked.