Amazon RDS – MySQL in the Cloud
This is an interesting development. The Amazon S3 system has been online for quite a while, and clients like Transmit and Cyberduck already talk to S3. But it's an object-database. Nice in many regards, but not exactly what someone building an application might be looking for. There are just way too many applications that need a general, reliable, relational database - like MySQL.
While I haven't used MySQL a lot, it's pretty much on par with the other open source databases - DB2, PostgreSQL, etc. It's a filesystem database where it's easy to backup the database by simply copying a few files. Nice, compact, and pretty good performance from what I've heard. I've used it in WordPress installs in the past, and it has always worked very well for me.
What is interesting is that Amazon seems to have broken down the costs so that you really are only paying for what you use. Be it CPU cycles to do the queries, or I/O to get the data to/from you machines. It's a pretty wild idea. If you couple this with the EC2 machines where you can bring up a linux box with your software on it, run whatever it is you need hitting this MySQL database with all the data your app(s) need. They even have a reduced I/O rate for transferring between their servers so that if you really can run the application on their end and have a minimal interaction with it, you're liable to be able to get away with a very inexpensive I/O cost.
Of course, this means that you're paying for the CPU cycles on their end... there is no free lunch. So it's an interesting option. If you need to have a cloud MySQL database, this is the only one I've seen. But the question becomes, do you need it? That's tougher to answer.