What is the dllhost.exe process on my machine that seems to use a lot of CPU?

A. DLLHOST.EXE acts as a host to execute any COM DLL outside a process address space. Microsoft KB article Q198891 has more details. 

With SQL Server this may be used for a linked-server/OLE-DB type connection to another machine or if you are 
invoking a COM object out-of-process using the sp_OAcreate stored-procedures.