Yes it is true. And this bug has just been discovered by me and confirmed by the Query Optimizer Project Lead (Conor Cunningham) himself, that i have the chance to meet at SQL PASS 2005.
He has been very kind and after I exposed the problem to him, after a little bit of suprise :-), in few days he confirmed that in some circustances SET ROWCOUNT, which is deprecated, is faster than the usually better SELECT TOP.
You can test it yourself, downloading the script right here. I must say that the problem only shows when you need to limit the rows...