The following link contains a tutorial on how to build a website in Illustrator:
http://aiburn.com/article/building_a_website_wireframe_in_illustrator

While I personally use Visio, the above tutorial shows its possible to produce a wire frame in Illustrator as well. While illustrator allows for a higher quality representation of icons, and images, Visio does provide multi-page support and so is an advantage when multiple screens for the same application are needed.

Edit:
One advantage Visio has is that it less demanding on the horizontal width of monitors. Most adobe products be they illustrator, fireworks or photoshop use up a considerable amount of screen estate for the right toolbar and that can be a hindrance for wire frames with their user specifications on the right.

I do like the idea of user specifications on the bottom of the page (printed out) since vertical scanning seems to preserve memory retention more than horizontal scanning.