I recently puchased a ceiling fixture for my new house (built 1946). The fixture came with a warning label advising to use only wire & connections rated @ 90degrees C & that most homes built prior to 1985 have wiring rated @ 60 C. I plan on using only a low wattage CF bulb. Can I use some 'shrink over' insulation to beef up the existing wiring within the ceiling box?

You didn't specify what type of wiring is involved with this fixture, but there is no accepted practice of "beefing up" wiring for a higher rating. If it is wire pulled through conduit, then replacement is relatively straightforward if you have patience and fortitude. However, if it were my house and I were using a fixture with a CF bulb, I wouldn't give the wiring a second thought. Compact fluorescents use relatively little energy and give of very little heat, so wiring selection is not a huge problem. This is of course my opinion.

Thanks Danleig. The fixture is replacing an existing one that has been burning a 100 watt bulb (15 amp cicuit)forever. There is no obvious heat damage to the wiring in the ceiling box, so I guess it makes sense that since I am actually reducing the wattage & amount of heat genereated I should be safe.
Thanks again.