It is related to the distance from the handlebars to the stear tube. Tiller means you swing the bars right to turn left. A mountain bike has zero tiller since the bars are directly over the stearing pivot. As you lay back the "ape hanger" handlebars on an old stingray, it starts getting more and more tiller. When you get to very long lwb recumbents like the tour easy, the tiller effect gets more significant.

http://www.easyracers.com/images/side.jpg

Some people are very sensitive, others seem to adapt with no trouble. If the bike design gets too long, most builders switch to remote linkages to eliminate the problem. The linear and longbikes do this.

http://www.linearrecumbent.com/linear_lwb.htm
http://www.longbikes.com/

Darren