Tag clouds are everywhere these days and I think that Jeffrey Zeldman had a point when he proposed that tag clouds are the new mullets. But, as he notes, "It is of course wrong to compare weighted tag clouds to mullets, mood rings, and similar instances of mindless pop-cultural detritus. Tag clouds are not dumb. Their smartness is why so many have rushed to use them."
Looking at this site's new mashup tag cloud I see there's certainly a utility value that may sustain it past the fad phase. It does help highlight where/how mashups are being used, at least in the current (unscientific) sample: mapping is clearly the most common with over two-thirds of the mashups relating to maps, geocoding or similar. Subjects like photo, shopping, and search are much farther behind, but essentially make-up the middle of the pack.
The mashup tag cloud is dynamic and will change as more mashups are added. It will be interesting to see how its shape changes over time. (See Wikipedia's brief tag cloud entry for a few more resources on the topic.)