In order to alleviate your confusion I will state for reasons of simplicity:
Get the Linksys USB100TX. It works perfectly for MSN v2.0 and is now well supported by Rasmus's current Linux image file as well. So right from the box to the back of your unit, regardless of the OS you will be running (I can't speak for QNX or others) it will work without too much trouble or headache. The device might cost about 50% of the current cost of the IA-1 though :) But I got this one because of the same concerns you have -- avoiding confusion and assuring myself of the best possible results.
There is a list of supported ethernet devices on the http://msnc.msn.com web site somewhere... maybe it's elsewhere, I forget where exactly, but it's buried within one of the FAQs and details an array of supported USB devices. The list is short!! The Linux list is longer, however, I would use a device that appears on BOTH lists to ensure the value of the device.