Yes, but when I use serial to talk to my Uno, the 8-bit is not set high. So putting aside whether or not the Duemilanove works, I think it is clutching at staws a bit to assume that there is a serious design issue with the Uno's USB-to-RS-232 converter chip that no-one else in the world has noticed.
I agree, there is no programming information passed from a running 328 sketch to the serial converter (8u2 or FTDI) about the format (speed, data bits, parity, stop bits) of the serial data. So please post as small a sketch as possible showing a serial data bit 8 difference/problem on serial data between running on a Duemilanove Vs a Uno board. Unless it's reproducible by peers (us) then it is just conjecture on your part in my opinion.
PS: If you indeed using direct register control for your serial functions rather then the standard arduino serial library then I'm sure there are those around here that can validate that it's being done properly or not.
Lefty