Things will never be the same—for society as a whole as well as for trade shows. Expect 25-50%+ attendance declines for many shows held this fall, particularly new ones that have not yet created a loyal base of attendees.
What should be your strategy for Fall, 2001 shows? First, seriously consider cancelling a show that does not have that built-in loyalty. The venues (particularly hotels) are being very accommodating. But realize that it will be extremely difficult to resurrect the show next year.
If you decide to hold the show, communicate immediately with the pre registrants, past registrants and prospects. Consider piggy-backing on President Bush's suggestion that we should all go about business as usual. In fact, you might use the line "They Win If You Stay Home" (or something similar).
But also play up the specific benefits of attending your particular show. Focus on the bottom line—how they can increase sales, save money, and succeed/survive in these tough times.
Good luck on weathering this storm.