This. It's less about scaling a framework, than it is about scaling your particular application.

If you find that MySQL is a bottleneck in particular, Ruby (and by extension Rails) has a wide range of database adapters and database ORM's available. Active Record itself has numerous adapters, which mean changing databases within the supported set shouldn't be too difficult.

Alternatively, you could use a different ORM such as DataMapper or one specific to the database you desire. Rails now let's you choose which ORM you wish to use, although if you're just getting started it might prove an additional learning curve as most of the documentation uses Active Record.