For all having this "desktop" issue on real desktops - please give us more information via hello @ snapbugz.com. Kudos!
My suggestion is instead of chasing UA sniffing (which is a losing proposition), just give up on blocking "mobile" or any kind of browser.
FWIW my mobile browser is just as capable as my desktop and as long as you don't block zooming I can deal with an "unoptimized" layout.
The creation part of snapbugz is just not ready for mobile/touch usage as UX patterns on the annotation part are different. However, the result / collaboration part is already finished: http://www.snapbugz.com/p/88a7a69a