ed 

Hmm...yes and no.

A longer top tube (I'm assuming you're referring to a Gary Fisher Genesis design) generally means that the seat tube is further back toward the rear making the rear triangle shorter putting more weight and traction on the rear wheel. Fisher's Genesis geometry uses a longer top tube with a shorter stem and a smaller rear triangle which will put less weight on the front tire and more on the rear tire.

So one would think it would make it harder to keep the front wheel down. I have owned 2 Genesis bikes and 5 non-Genesis bikes. I totally prefer the climbing ability of the Genesis bikes because I feel a little more in control and more importantly I get more traction. I can always lean forward over the bars more if I want to keep the front end down but don't need to do so. I don't EVER feel as if the front end of my bike is too light on steep climbs.

So I guess it could be said that a short top tube puts you closer to the front of the bike over the front tire, but I don't feel from my own experience that this is an advantage when climbing. More of a detriment because of the loss of traction to the rear wheel. Generally though, back in the formative years of mountain biking, you would see stems that were like 160mm long and layed you out like you were riding a road bike. It was good for climbing as long as you have adequate traction, but it threw off the handling of the bike on descents and technical segments...trying to horse around that log of a stem.

If you're concerned about the climbing ability of a certain bike that you're looking at buying, by all means tell us what you're pondering and we can give you our opinions.

Or better yet, tell us a price range and we can make suggestions. I am a big fan of Fisher, newer Giant's, Trek, and Specialized because that's what we have a ton of in my area. Parts are readily availiable if there's any breakage and they all have models that perform well.