Today i get a question from my colleague. "Is it possible to upgrade WSS 3.0 to Search Server or Search Server Express 2008 on the same machine?"
After some research I found the answer: Yes, it's possible. (Link)
But, as next I get the question: "May it's possible to bind WSS 3.0 to Farm1 and bind Search Server 2008 to Farm2? (WSS 3.0 and Search Server is installed on the same Box)"
Why: Search Server and WSS are part of the Office Server platform.That platform has tools in the 12-Hive folder like psconfig and stsadm. Psconfig is used to bind/join WSS or Search Server in a farm. In psconfig it's not possible to select WSS to join in farm1 and Search Server to farm2.
The limit is you can bind/join one Office Server installation (including WSS, MOSS, Search Server,... all on one box) only to one farm.
I hope that help for further research.