S Crisp,
I use a Nikon ED50 with a monopod, but only for specific habitats. Like looksharp above, I think it works because the Nikon is short and light and I find it works best up to 20x magnification. 27x is often still very useable, 40x is still useable but only for a quick look. However, for any birding that involves standing for an extended period of time in one place e.g. scanning a reservoir, or a large flock of roosting waders I almost always take a tripod. I wouldn't even think about using my 80mm scope on a monopod. However, I suggest that my experience is no more or less valid than captaincarot's, because it all depends on where you personally draw the line between useable and unusable, and that has a lot to do with the type of birding you expect to do.
I therefore recommend that you take your scope to a "Field Day" (have a look at the In-Focus website for starters, but there are others) and try it out with a monopod before you buy one. If there are no convenient Field Days then your local Jessops and/or other camera shop is sure to have some monopods to try. If you can stand the embarrassment it's worth trying it outside the shop in daylight. Hopefully it will be a blustery day, which is your worst-case scenario, because theroretically a longer scope has more area to catch the wind, and more leverage for the wind to knock it about.
Regards
Mark