I gave the Player (2.0.5129.5191) a try on a Windows Mobile 6 device (iPAQ 214 - 210 or 212, different regional references for the same device).
The good news: it installs fine, it detects a bluetooth GPS (even in a software split port); overall it seems to work.
The not so good news:
when it does not find a cartridge in the configured path it either: a) crashes; gives a loop notice (nagging, one warning would be enough and afterwards we should be able to change settings/the path to the cartridges etc..; as it is one has to be really fast to close the pop-up warning and quickly touch the "menu").
Graphically it does not adapt/cope well with the VGA (480x640) resolution (see the screen shots bellow): the GPS fix icon is small and oddly surrounded in white; the fonts for the cartridge title do not seem to have space for rendering; the arrow showing the direction is too small and what I guess is the distance indication to the left does not show.
Sadly I have no nearby cartridge to further test this but as I create and test my own I intend to update-report this post with my "findings".
Anyway, Wherigo is a step in the right direction and I am very impressed with the features in the Builder.