Any time you are trying to cram multiple channels thourough on medium (satellite, cable, internet) a compromise must be reached between quantity of channels and picture quality (compression). To add channels, a satellite provider can either compress more, or add bandwidth (more transponders and/or satellites). A cable provider has the limitation of a single coax or a single fiber - but as time progresses, more and more bandwidth can be crammed onto one wire. OTA signals need not be compressed much, if at all since the bandwidth allows a nice picture.
Satellite providers will tend to compress and get as many channels as they can without losing too many subscribers due to poor picture quality. With the move to HD, providers will need to add bandwidth or improve compression. You can count on them doing both. They are putting a lot of hope into MPEG4 allowing more compression than MPEG2, but from things I've heard, MPEG4 is not a cure all.
Cable companies can cram a lot more on a cable given the necessary hardware upgrades. They can also eat more bandwidth by lowering compression and increasing picture quality - easier than satellite providers can. The cable
companies will continue to invest in beating satellite picture quality, which will drive their costs up. Satellite providers will have to launch more satellites and raise prices. Both can use MPEG4, but it won't really accomplish much over MPEG2.