While looking into some data quality issues, I considered the case of source data arriving as a string of numbers with embedded commas (for example, "1,526,734.56"), and as I didn't have any such data to test with, I decided to create some, and also created this function in the process (which can be used to create some): fn_AddCommasToNumberString.
(Followup on 4/23/09): A forum post on SQLServerCentral.com explained a very easy way to do this using varchar conversion:
declare @test float
set @test = 7265342.12
select @test, convert(varchar(20),cast(@test as money),1)