So I've looked at heroku a bit, and I have to say it's pretty promising. I assume eventually they'll allow you to run your own domains off of it, maybe offer you email, etc. I actually somehow didn't meet the guys at the first Y C dinner, but I think they're going to make a lot of money.

The problem is that it's only one company. Though I'm sure they'll have clones if/when they are successful, the barrier to entry there is a lot higher than simply supporting a language like PHP on a shared hosting system. (I suppose I'm only guessing here, as I've no experience with either building a Heroku like site or operating a shared hosting one, but the sheer numbers certainly imply that.) So there are always going to be 100x more hosting options for a newbie PHP developer, even if the Heroku model catches on.

I chose PHP because I found scores of web hosts for it, and I knew that if I ever wanted to switch, I had options. Also, I think brand new programmers (think high school kids) get in largely by Googling for web hosts, signing up an account, then coding in whatever is available. If 95% of the web hosts they look at are touting PHP, that's going to play a large factor in their choice.

Not to mention that Facebook, Yahoo, etc. are in PHP, as you can easily see by looking at a URL, and that no really large sites are in Rails, and even if they were you wouldn't know it at a glance.