Unfortunately yes I do think there will be more acts of terrorism visited upon the US.
Part of teh shock of 9-11 was that it happened on US soil. Suicide bombers are a daily threat elsewhere and teh US has bee left relativelt unscathed from such things with the exception of domestic terrorist activities (OK City).
It is this domestic threat that is more likely to escalate in North America than foreign threats IMO. I do really feel the US needs to concentrate more on domestic issues before continuing their world police function they had assigned to themselves.