How scalable is MySQL?
Well that of course depends on your needs, but let me give you some examples of real-world scalability. At the 2005 MySQL User Conference last month I encountered users running MySQL on 20 terabytes of data for statistical analysis. I attended sessions where Ticketmaster described how MySQL was scaled to handle over 500,000 ticket sales per day through replication setups of dozens of servers in a multi-location replication hierarchy. Wikipedia uses MySQL servers with up to 32gb of RAM to handle high loads and MySQL is capable of handling even more memory and resources. Big names like Yahoo, Google and Slashdot use MySQL to run high-load Web applications.
In the end it all depends on what kind of scaling you need, but you can rest assured that MySQL has scaled very well for others.