I can confirm the lack of noise with UM1s on the 5g, at least on the 60g model.
The only complaint I would have about the rockbox'd 60g with the UM1s is the pop it makes when it is just starting up. Though I avoid that by not putting my cans in before I start it up.
I think a 5g + UM1 is a great combo and i would suggest it to anyone. I liked the sound on my H300 better, but I willingly sacrificed that for the storage and size of the iPod since they have been running Rockbox.