If you have unlimited hardware at your disposal, work on projects where a slightly faster development for certain, specific cases can be obtained, or work on projects of a size that can be managed by the performance of a single machine, sure. If not, R/R isn't replacing PHP.
Rails doesn't really need that much hardware, that's a very common misconception. PHP is not the fastest thing in the world either, unless you go with a compilation cache server. Rails can be easily perform well under single machines. I have VMs with 512Mb of ram handling thousands of users per day with Rails with no problem.
Of course, it's not like a modern first person shooter or anything. My point was that on one and the same machine, PHP gets a whole lot more done per portion of computer power than Ruby does :)
