If you know you're going to scale your application well beyond what a single server can handle, then virtualization isn't obviously required. It can still be really nice, for server recovery and such. The downside is the overhead, when you distribute a number of networked processes over multiple VMs you slow them down a bit.
A 'classic' scaling story for LAMP could look like this:
First 1 server with the full LAMP stack directly on the one server.
Then 2 servers: 1 for web serving; and 1 with plenty of RAM & disk I/O for MySQL.
Then 4 servers: 1 for load balancing with origin IP address based sticky sessions (and optionally static file serving, SSL offload); 2 for web serving; and 1 with plenty of RAM and disk I/O for MySQL (and optionally a warm standby backup MySQL server).
In this progression, I don't see virtualiation as that useful. Oddly enough, after the 4 servers, virtualization becomes more relevant again -- the next step could be cloud computing.
But there is another thing to consider -- what about about auxiliary servers like mail servers, monitoring server, package installation repositories, etc. Do you need any of those? If so, they could be good candidates for virtualization, as they probably don't require a full servers capacity.