Jon Gruber points a piece the troubling news that you can't sync read-only iCal subscription calendars to your iPhone, as confirmed in Apple knowledgebase article TS1213. What's worse is that Apple took this feature out of their US$99 subscription service, but it's still available when you sync your iPhone (for free) with a cable. The problem is that they're mutually exclusive so if you use MobileMe you lose the ability to sync subscription calendars.
This massive ommission in MobileMe isn't a big deal to me for calendars related to holidays, sports schedules, movie releases — full list at Apple's calendar page — but it's a huge deal if you subscribe to calendars published to the Internet from someone else using iCal. Especially for families or businesses that maintain and share several iCal calendars.
So you can sync read-only calendars to your iPhone with iTunes syncing, but with MobileMe you're out of luck.