For Apple to gain marketshare it has to improve on the hardware side. I think their software side is nearly there. They still need a bit more for the enterprise. (i.e. SAMBA still needs more power, and I'm told there are a few other areas they need improving on, such as their Java VM)
If Apple comes out with a cheap, headless low end Mac, I could see their marketshare improve. Although I doubt it will be more than a point or two their first year. However I'm not sure they'd want to cannibalize profits and do that. While I know there is a new iMac coming out, presumably with a new design, I doubt it will reach out to a marketshare radically different than what the current LCD iMacs do.
Panther certainly is sweet though. If 10.3.5 or whatever has the BeOS like Finder/FS ala Longhorn and some of the other UI advances that are rumored come to pass, then they certainly could improve greatly.
The other big issue is how the PPC keeps up with AMD and Intel. Right now while the dual G5 is sweet, the price/performance isn't quite what it could be. It would be nice if they hit 3 GHz soon. But I doubt it will be before June.