Casino Niagra could be an inviting target for a truck bomb. True, it's on the Canadian side but is very popular with Americans due to the favorable exchange rate and lack of witholding tax on winnings. Besides, Canada was declared a target by Bin Laden due to our (ongoing and substantial) involvement in Afghanistan. I don't think these jackals really care who they kill in the end.
There's the Mohawk power generating station. Knock that puppy out and you have a major disruption of the whole NE power grid that could make the recent blackout look like a picnic, and don't forget that it's cold up here this time of year, unlike during the balckout where the weather was quite pleasant.
The other possibility is that instead of being about Niagra it could be a signal to a cell based in the area to execute a previously planned attack. I would hope U.S. and Canadian authorities are watching the border crossings very closely (and the two forces can and do communicate in real time).
Another option is that the image of the falls is symbolic of something else entirely. One of the messages reported on during the whole "get out of NYC/LA/Washington" period cryptically kept referring to "the water, the water." In this case the visual of the picture could be seen as matching the text of the message. Water could of course refer to ports, coastal cities, whatever.
Keep in mind that the dead terrorist referred to in the most recent Bin Laden tape lived in St. Catherines, Ontario for a time, and St. Catherines is a stone's throw from Niagra. Where there's one cockroach there's likely to be more if you look hard enough.
While we should laugh at the fear they want to cause us, we should moderate that with the fact that we are still a target, albeit a harder one.