There are four PSP models right now- the 1000, 2000, 3000 and the soon-to-be-released PSPGo.
Rumor has it that Sony may be planning to release at least eight different PSP model revisions according to PSPGen (linked above).
While investigating PSP Firmware 6.00, apparently an index_0xg.dat file in it can load from _01g to _08g... which may mean support for eight versions.
Of course this isn't concrete evidence as the modules could simply be associated with prototype or debug versions, but don't be surprised if we end up seeing another PSP revision next year.
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