I could be wrong but to my knowledge there is not.
windows, iphone, symbian, android, RIM, WebOS... although I haven't had first hand experience with RIM or WebOS I don' tthink any of these really compare to the n900.
Best comprimise in my opinion is android (although not 100% sure of the media capabilites)
For future upgrade, depends when you want to upgrade, meego device was meant to be this year sometime (although I haven't kept up to date with meego device news)