Process context of Plugin execution

If anyone can answer this question, I would seriously appreciate it...

We have a CRM 4.0 installation which uses two servers, one for the 
application, and a dedicated sql server for the databases.

When Plugins are registered using disk, GAC and database storage, which 
processes (in the above context) on which machines are used to execute the

Plugin code.

I would presume that they would all be executed within the worker process
of 
the web application, but can anyone verify/clarify?

Cheers