As for why someone would choose MySQL, there are a bunch of reasons. There are a lot more people with MySQL experience out there. MySQL has better replication facilities (and I've set up replication with MySQL, Slony-I and PgPool-II). I'd really like to see Mammoth Replicator become the standard in the PostgreSQL community (as well as for 1.8 to be out of beta) as I think it's a considerably better replication design than the other options in PostgreSQL, but right now MySQL replication looks a lot better. Maybe you have a good use for one of MySQL's less used storage engines. MySQL Cluster looks interesting, but I wouldn't trust my data to it today (even if Zillow seems to think it's the best thing since sliced bread).
The differences between the two are really minor today. Choose whichever one you like, but there are definite reasons to choose either one.
2) I am not trying to slam MySQL. But seven years ago, we had one database which was not ACID compliant and had inconsistent behavior, but was very fast. And we had another which was designed and built properly from the ground up - with the future in mind - but was quite slow. There were advantages and disadvantages to each one. But over the past seven years, PostgreSQL has had time to optimise and stabilize code, and as a result it performs just as well as MySQL in most cases. Whereas in the past seven years, MySQL has worked to implement the essential features that allow it to be a robust database. If you were starting a website in 2009, why would you pick MySQL? I could easily understand why in 2001, but not now.
3) My point is that the "Disks > RAM > CPU" argument is no longer valid. The point of that statement was that you should spend your money on disks first, then RAM, and focus less on CPUs. This is not true in 2009! Now (for web apps), you can easily buy enough RAM to cache your entire database. All of a sudden, RAM and Disks are no longer an issue and your DB becomes CPU limited. All that discussion about "more spindles = better" and "raid 1+0 > raid 5" is not very important anymore; you just need a disk that's fast enough to log your transactions.