You can file tag clouds (heat maps, weighted lists, folksonomy lists, whatever you want to call them) under 'cool concept, but not very practical'.

A cooler concept might be implementing an XHTML micro-format for weighted tag data. That would at least give individuals (and bots) a fighting chance of processing/displaying the data as needed.

As a side note, these tag clouds are not very accessible either. Here's a more detailed analysis of some of the shortcomings: