Currently, for desktop PCs, their market share is probably in single digits. For laptops, they may have a higher market share. For media devices like the iPod, Apple has a very high share. But market share doesn't tell the whole story. The 'installed base' share for Apple is higher than their market share because Macs typically have a longer lifespan than PCs and aren't replaced as frequently.
I think Apple will gain some share in the traditional desktop market. The new iMac is close to perfection as a desktop computer.
In the future, home PCs will be moving to the living room, attached to HDTV sets. Apple is well-positioned for growth that market.
"For media devices like the iPod, Apple has a very high share."
I'm not very up on current gadgetry like the iPod. They don't use OSX in that, do they? I would imagine that would run some highly simplified (ie. stripped down) embedded OS.