From Ajax Patterns
I would probably tie this into a brief discussion of the problems with multiple requests (related to the fact that if no attempt is made to tweak the percieved performance of the application then they will assume things need another click).
Perhaps tie this into the "Synchronizer Token" pattern on the server side.
In terms of other relevant patterns on this site, I would have thought a combination of this with throttling might be an interesting paragraph or so.
Another interesting idea for certain types of calculations might be relating to keeping track of a history of how long requests have taken and query that (although this is server side). A query on how long it is likely to take is then just a call to help the progress bar be a bit more accurage, or to perhaps give a rough estimate in writing to the user (e.g. "it is estimated your query will take 30seconds"). So that would improve the accuracy of an estimate coming back.
PS the progress bar suggestions sound very microsoft-ish ;). What about the scroll up then back bar?