For registered Marketplace developers, here is a handy tip via Nanapho.jp.
An App Hub account usually only allows developers to developer unlock up to 3 devices. However a simple request using the App Hub support form can increase this to seemingly arbitrary numbers.
In fact in an enterprise setting it has been possible to request up to 100 devices to be unlocked so far, which for a small company would allow vertical applications to be distributed as a trial quite easily.
Read more at Nanapho.jp here.