In other applications I use where there is a desktop version as well as mobile ones, a user is allowed to have a desktop and a registered mobile device open at the same time. I understand the reasoning behind limiting the number of signons from a id-sharing problem, but if a user registers an iPad or iPhone, the multiple signon/multiple user issue should be able to be managed and would allow those of us who are mobile to have our system logged in on the desktop, and when we have to leave it, still monitor the market without worrying about logging out/logging in multiple times. And I fully support the one desktop logon at a time.
I used to own a software company and am sensitive to the problem of non-registered users accessing the software and products, but it can be managed.