Thanks guys.
As I said, its not a problem I envisaged having but rather one which the client had encounted and it obviously left a nasty taste in his mouth.
Given that an expensive query cannot be persuaded to complete in a reasonable amount of time with a bit of tweaking and a good dollop of hardware, I think I'd plump for a hybrid of these suggestions. That is to track requests to expensive pages by user and not allow a user to access the page more than once.
Following Andrews suggestion, I had contemplated the idea of allowing the user to kick off a query and then sending them notification via e-mail or sms when its ready to collect - probably with a link to a result page or a file to download. Another option is to send them the result as an e-mail but this would only work well if they accepted html content e-mails which I'd rather not force. I may persue this out of curiosity if I get a mo.
If you guys are not experiencing a problem, and you have been runnning OpenACS sites far longer than I, then I suspect its not the problem the customer makes it out to be. Of course, these words may come back to haunt me :o)
Thanks
Steve