I think you nailed it. Large enterprises want support. Although some also see potential cost savings of Linux.
Medium enterprisey project with a medium budget, I see people picking C#/CLR>
But really big enterprisey stuff with really big budget? It's all Oracle DB + Java on Unix there.
Just as all the really big infrastructures (Google, FedEx, Amazon, etc.) are all Unix.
If i were to do a big enterprisey project with a big budget i would definitely be more likely to go with windows while if i were to do a web/online start-up i would more likely pick Linux.