I've been dealing with this issue too, whether or not to take a guitar. I play on a daily basis now but will be out on the road for at least a year on a RTW trip. I've seen that Martin makes an travel acoustic they call the "backpacker" that can be picked up for less than $200 USD, but haven't had a chance to play one. Supposedly they're less than 3 pounds. I was wondering if anyone here has seen them on the road or had a chance to play one.
I was in the same predicament for a while... Then I caved in and bought a mandolin! As far as string instruments go they're about the most travel-friendly I think!! And great fun.
As has been mentioned on here, the travel guitars with the significantly reduced body seem like a good way to go for about £100 new. (Still not as small as a mando though! )
I have been traveling for over 3 years now with the lapstick , it is an electric travel guitar you can use your headphones or normal amp. Flying regular from far east to Europe sometimes i get up to 6 hour playing in the plane, normal econonmy class seat. i tried some other travel guitars which were good but this one is just different and suits my purpose better. They have a web site www.lapstick.com