Making simultaneous releases for Mac and PC is an investment, but oh what a sweet investment it is. Here's some good information on simultaneous releases:
1) You need to have a portable code base. The more portable the code the easier it is to release on both platforms.
2) You need to plan to port the code early in the development cycle. Waiting for PC sales is a cop out because by that time, you miss all the "early adopter" sales that go along with game releases. Now all you have are the casual mac players but without a near simultaneous release, you have lost sales, period.
3) Mac users in general are 5 times more likely to purchase software than PC users. Mac users are far more prevalent at home than at business, and game manufacturers simply cannot target businesses. Especially when it comes to games, the PC to mac market share percentages are the wrong figure to look at.
4) Respect Mac users, like any user, and they will respect you.
5) Parity parity parity parity!
6) Do market research ahead of time. All this waiting to see how PC sales do will not save you any money.
7) Learn X-code!!!!!!! It's brilliant and comes with each and every 10.3 install CD.
8) Don't just do this with one game, establish a business around it, just like with any long term investment, and learn to streamline it. The more time you spend on a good process, the better it gets.
9) The attitude your employees should eventually have is "Duh, of course it works on the mac, too." The process should be well defined, automated, streamlined, and not too complicated.
10) They did this years ago with the apple IIs, commodores, TRS-80, and all the other computers of the age, you can do it now!
Of all the things Blizzard does, they do ports the absolute best. They may have problems with design balance or deep nasty bugs here and there which break functions, but they have no problem keeping mac and PC versions on the same level.