Ramblings from the Creator of WilsonDotNet.com
Paul, whilst I've not run in to this problem it would be benificial if you could explain how you "just create it once and reuse it" - just in case I (or others) run in to the issue. :-)
Create the serializer once, and keep a reference to it.
(sorry - I should have mentioned that my previous post was in answer to Phil's question).