Take a look at these:

PreSonus Audiobox ($150) http://www.presonus.com/products/detail.aspx?productid=53

FiiO E9 ($110) http://www.amazon.com/gp/product/B004M172FY

Peachtree Audio iDac ($1000) http://www.amazon.com/Peachtree-Audio-idac-Digital-Converter...

or the Nova for just a bit more and it has a tube and built in amp: http://www.amazon.com/Peachtree-Nova-Integrated-Stereo-Ampli...

The latter is what my cofounder has (pictured in my post in the background on the window sill) and loves. He also had a Decco for a while (before realizing they were underpowered for his speakers): http://www.youtube.com/watch?v=SljFut8cUnc&feature=chann...

A few more that I'm not too familiar with: http://www.amazon.com/Audiophile-Products-Fubar-Plus-Headpho...

any usb dac should work on linux; they appear as standard audio devices (and this doesn't depend on the make much - there are bunch of standard chips that everyone uses as usb interfaces). certainly the musical fidelity v-dac does, but that doesn't include a headphone socket (they make a separate headphone amp). if you just want something simple, i'd suggest a nu-force icon. if you want something obscure and boutique, have a look at audio-gd... if you want the latest technology check whether it supports "asynchronous mode" (audio-gd don't, generally; music fidelity v-dac ii (just released) does; icon doesn't; you can also buy "digital interfaces" that do async-to-digital, like the mf v-link) - they are supposed to reduce jitter (timing errors).

i am not guaranteeing you will hear any difference between any of the above ;o) i own the v-dac (i), which i feel improved the sound of my logitech duet (less abrasive), but that is feeding an amp and speakers, not headphones.

also, on music while programming:

- i find that if i need to concentrate, the music just disappears into the background. it's not like i need to switch it off, it just becomes irrelevant.

- i wrote some code that uses last-fm data to construct playlists of related music. if you have a lot of mp3s and want to listen to "random but related" music, this works great. it's not packaged nicely, but if you're used to hacking python you should be able to get it working - http://code.google.com/p/uykfe/ (as well as generating standalone playlists it talks directly to logitech's squeezecenter to dynamically select music).

- i may be showing my age, but fleetwood mac's rumors or nirvana's nevermind are my go-to albums for difficult bugs.

There's a good list here: http://www.head-fi.org/products/category/amp-dacs

I was eye-ing NuForce Icon (can't remember which exact model) about a year ago (have dropped this requirement since)