That is a good question George and one I cannot answer yet. I have not tested running 32bit SQL with database mirroring on a 64bit machine. Though the 10 database limit is only a suggested limit as I have a 32bit SQL Server on 32bit Windows mirroring close to 30 databases without issue.

Now the databases are extrememly small and there isn't alot of traffic flowing in and out of them but you can successfully mirror more than the MS recommended 10.