Just to be different from the other replies... I chose the SE model specifically because of cruise control. My last three cars had cruise fitted and my next three will have cruise fitted. I find trying to keep to the speed limit especially in 30 or 40mph zones near impossible because the engine is so flexible and quiet. So instead of having one eye on the road and the other on the speedometer, I stick on cruise - problem solved. I don't care if a sign says speed camera's ahead, or a patrol car appears from nowhere - I know exactly what speed I'm doing.
I'd also argue using cruise in built up areas is safer than driving without cruise. Normally when you do an emergency stop, unless you're Jenson Button and use left foot braking, you lift your right foot from the accelerator, move it over to the brake, and press as hard as you can. However with cruise control, my foot is constantly over the brake pedal... I'll bet I can stop quicker whilst using cruise control than when I'm using the accelerator.
However I would have thought the cost of retro fitting cruise would be prohibitive. Even if there was an official Volvo kit is available (there's a Volvo aftermarket computer kit available - you even get a replacement left side steering stalk), I'd have thought the sticking point would be labour costs.
There's the non-Volvo route. Sites such as
www.conrad-anderson.co.uk may give you some indication of prices. But then you may have to consider that a non-Volvo approved fitment could invalidate the Volvo warranty? It'd probably be worthwhile checking that your insurance would remain valid as well. You may have to delcare to your insurers that you have this fitted.
Is that enough to put you off the idea?