i've been MTBing on well built 36h wheelset and weigh 30# on you... no problems... I did however manage to kill some some spokes on my redline on my last road ride (62 miles on tour de cure)... but I think that has to do with me landing a jump (being a jackarse laughing at the skinny tire buddies haha) also could have to do with the fact it's factory built wheels...

could have it rebuilt with decent spokes, hand built, proper tension ect... also a bit wider tires would cushion the ride a bit and posibly help the spokes live a bit longer