Gaming is a special case because so many games come out for the PC that never come out on the Mac, the biggest example being the Half-Life series. There's arguments that this could revive Mac gaming because it would make it easy for PC game developers to release Mac versions. There's opposing arguments that nobody would develop games for Mac anymore because they can just develop for Windows and have Intel Mac owners.
Personally, I have more faith in Mac game developers than to think they'd just abandon all the PPC Macs out there. The major Mac game developers (Aspyr, MacSoft) have already gotten in line behind the Intel move. I'd assume Blizzard would follow suit. In the end, I think the move will be good for Mac gaming. Half-Life probably still won't come out on the Mac, but future games that wouldn't have been ported to PPC might come out for Intel Macs.
Outside of gaming, I think there's no problem at all. Apple has made it ridiculously easy to port PPC apps to Intel, as long as you use Xcode. The gap between the range of apps available for Windows and the Mac OS is nowhere near as large as it is for games. Sure, there are more apps out for Windows than Mac, but a lot of them are duplicates and the Mac OS has a large shareware/freeware community. Those small developers will make the switch with little trouble, I'm sure.