Comversing (computer conversing) with the LINT gang on tag clouds, I did a quick hunt to get some design ideas.
I found this interesting example that uses the text of US presidential speeches starting with John Adams in 1776 and finishing with George W. Bush in January of this year. Not only does it use font size to show frequency of words, but it also uses font color to show whether the words are newer or older.
It has a slider so you can view the cloud over time.
Details of how it works are on the site, and the generator used to create it is available for free under a Creative Commons license.
It loaded a little slowly, but it was worth the wait.