The iRiver U10, as you might know, contains FlashLite 1.1, so one can upload Flash Lite content onto it.
Apart from the question whethere there’s a market for FlashLight applications on mobile media players (I guess it’ll come down to how many funny Flash clips will be released) there’s of course always the option to learn something new (and maybe exciting).
In the case of of FlashLite, there’s the chance to learn through Macromedia/Adobe’s Mobile Devices & Dev Center which even contains a sub-sidde devoted to the iRiver U10.