Mike (TT) indicates the primary factor is the amount of clothing necessary to keep warm and the constraints on your motion.  I'm in the 80's club speed using AP1s.  So long as the ground isn't frozen, I don't have a problem playing into the 30s.  One of my regular partners is north of 100 mph, uses MBs and ProV1x.  He finds it painful to hit irons a little off center when the temps get down to mid-40s and lower.  If you get too much feedback under a certain temperature, you can try NXT Tour-S.  I also rotate a ball with each hole so each ball coming from my pocket is a little warmer coming off the tee.  Actually just had my best round of the season in 46 degrees this past weekend.

I just wanted to jump in with some additional clarification on this. Here is the feedback from our R&D team when asked the question below...

“Will temperature effect either of these golf balls in different ways?”

R&D: If you are playing with cold golf balls you’ll see distance loss. We recommend playing with room temperature golf balls. However, the other factors that typically accompany cold-weather golf (i.e. wearing more layers, frozen ground, wind, etc.) might have a bigger impact on a golfer’s overall performance.

You can check out the full Q&A with our R&D team here: http://www.titleist.com/teamtitleist/b/tourblog/archive/2011/03/29/titleist-golf-ball-r-amp-d-team-answers-your-questions.aspx

Hopefully this helps answer your question.

Cheers,
Mike 

OK. So as I drive to the course I place my sleeve of balls in the defroster slot on the dash, and they go from room temp to slightly warner., Then i keep two in my pocket, and change after each hole to the wamer ball. Granted they do not stay warm but not as if they are freezing cold. If it is cold enough to use a cart warmer and cover i make provisions to keep a ball warm or two. I personally do feel the difference and believe it does help. Is it only just a feel thing, or a performance thing? How much would R&D say this helps as far as distance and spin, etc?

We are the die hards... usually 40 degress or above but occasionally it might drop on us... cant miss our golf... Tennessee,,,  we can make it year round mostly,,,