It's adc is 5volt and the only difference is how they connect to the pc the uno uses a atmega16u2 for USB to serial and duemilanove uses a FT232RL and only has 5 volt power uno has 3.3 at 50ma and 5 volt
duemilanove also provides 3.3vdc at 50ma to the shield power connector, it comes from the FTDI chip.
From a writing sketches and wiring up to external components the duemilanove and Uno are 99.999% identical. Only difference is UNO has a slightly smaller bootloader so duemilanove has slightly smaller flash size and uploading sketches is twice as fast on the UNO. Eith board is a great way to get started learning about arduino programming.