Usabilla is one of the remote, unmoderated usability test services I've reviewed and used in the past (full disclosure: they also now sponsor this site). They've added mobile testing to their remote, unmoderated testing service. The new feature lets users participate by using their smartphones or tablets to test the screenshots or urls you specify in each task or scenario.

As with their past offering, participants are stepped through each scenario and asked to respond by adding markers to screens and notes/responses to your questions. I ran test scenarios on both iPhone and iPad, and the interface is clean, and simple, and optimized to work well with both devices. Read more here.