Head tube length is a very important dimension, since it really does tell you the true vertical size of the frame. What's actually more relevant is the vertical distance from the center of the BB to the top of the head tube. A number of companies are now listing this dimension om their geometry charts and call it the stack or stack height. This dimension eliminate errors in fitting that might occur from differences in the BB drop. A few brands, like Serotta use an 8cm BB drop on some of their frames and that allows the head tube to be 1cm shorter than a frame with a 7cm drop and still produce the same saddle to handlebar drop (with the same stem setup).

You also have to consider the headset stack height. Frames that use conventional press fit threadless headsets will have a headset stack height of 25-35mm, while a frame with an integrated headset can be as little as 8mm. The frame using the conventional headset can have a shorter head tube length, but still produce the same saddle to bar drop.

Fork length also affects the head tube length. Road bikes use forks with lengths ranging from 360-375mm, while cross bikes use a length around 395mm, so the head tube would be at least 20mm shorter and still have the same stack height as a road frame (if the BB drops are the same).

http://www.cervelo.com/en_us/bikes/2011/R3/geometry/