I received a similar offer, but with a lower threshold: $2500, which should be easily achieved. Mind you, FirstUSA *still* hasn't posted my bonus BA miles for the Visa account I opened towards the end of the summer...
These offers are probably targeted - the gardener also has a BA Visa, and has yet to receive this offer. Could be that First USA is somehow tracking our spending on other cards - Diners gets a healthy chunk of business from me every month.