One of the big things you'll hear about Ruby on Rails is that it's hard to scale. In my opinion, that's not really an issue that one should be addressing on the framework level. Scaling, in my opinion, should be something reserved for backend databases and servers. From what I've read, you should be able to slap on a MySQL proxy and an Apache load balancer in front of multiple mongrel servers, but I digress.
Will this make me abandon my quest for learning Ruby on Rails? I highly doubt it. Scaling becomes an issue on extremely high traffic sites. And a lot of the issues that come into play with such a high traffic site, can be easily remedied by things like optimizing queries and caching.
Source: Twitter Said To Be Abandoning Ruby on Rails [TechCrunch]