If you're mounting by right-clicking the file and selecting 'archive mounter', then I've found that you won't be able to play them as DVDs.
VLC should be able to play it, as mc4man wrote.
Alternatively, in terminal, you can
(You only need to create the directory once)
sudo mkdir /media/fakedvd
sudo mount -o loop path/to/movie.iso /media/fakedvd
Ubuntu should then act as if you have inserted a DVD, you'll get the prompt of what to do. It treats it entirely as a DVD, including preserving the DRM, so you'll need the DVD decoding stuff <--this is probably why you can't play the ISOs straight in VLC etc.