For a community site, I'd go with InnoDB over MyISAM. First, when your cache does fail over, your database will need to deal with high read-write concurrency, which InnoDB will be better at. My second reason is that you'll get transactions, which if you need to update more than one table at once, will be a good idea. The only thing I could see using MyISAM for is full text search, but you might be better off going outside of MySQL to solve that problem.

As a side note, I've seen a lot of people over-build systems based on theoretical scaling numbers that their sites might never actually see. It's pretty hard to grow a community site from scratch to 24M hits/day. In most cases, that kind of usage doesn't sneak up on you, but grows gradually over time. This will allow you to grow your solution over time as the usage patterns and growth rates emerge. If you're small, stay flexible and get a good solution out quicker, rather than building for a goal that might be years out. Obviously, I don't know your situation, but that's my two cents.