Here is what you do:
Open stock browser address bar, type about:debug and press enter, nothing will happen but new settings are available, including user agent
Only one thing I have noticed is youtube still goes to mobile site, not sure why ?
Edit: seems it is not so permanent as it has reset after I restarted browser although saving a page to bookmarks whilst in desktop keeps it that way.
After some more reading about this, it seems not possible on stock browser, but luckily it is with chrome with a data/local hack.
Edited by jikobutsu, 21 December 2012 - 02:58 PM.