Not a huge market share no, but they're used in thin clients / signage / point of sale / etc.
Btw, it's quite interesting to have these things happen, just like once the 486 patents expired 486 clones started appearing from non-intel-amd-via manufacturers. 586 and MMX are free to go now it seems, and we're seeing 1GHz low power cpus with those.
curaga, do you have an Xcore86 cpu yourself? It is just difficult to maintain pure legacy MMX code and make sure that it always works properly (no stray SSE-enhanced instructions accidentally mixed in) when there is no hardware available to run tests.