Now, Hotel City, the newest of Playfish's Facebook games, has announced an offer where simply leveling up can give you free PF Cash.
Right now, getting to Level 10 will earn you 4 PF Cash, and getting to Level 25 by April 12th will earn you another 4 PF Cash, for a total of 8 PF Cash -- that's about $1.60 in US Dollars.
While this isn't much, it's nice to finally have a way to earn premium currency for premium items when many other games already have this option.
Keep in mind though, the PF Cash won't be credited to you until April 16th. There's also no guarantee yet, that you'll be able to use the PF Cash to buy stuff in games other than Hotel City, though given that PF Cash is the same in all the Playfish games, it's a pretty safe bet to expect that you can.