Will an attack on Iraq increase the threat of terrorism?

I tend to agree with you. Islamic terrorist organizations do not need special reasons for attacking the Western countries, first and foremost the USA and Israel. Their interpretation of jihad implies attacks on the "infidels" for the very fact of their being these. Of course, they may use the U.S. attack on Iraq as an excuse for their PR, but this will be just the same type of excuse the Palestinian terror organization used to start the current uprisal, explaining this with Mr. Sharon's visit to the Temple Mount. The current outburst of violence was planned for many months, and planners could not know in advance when Mr. Sharon was supposed to visit the Temple Mount.
The same refers to the activity of al Qaeda and the similar organizations. They plan terror attacks all the time, and they perform them when they have an opportunity. And linkage to any actions of the country they acted against is nothing more than a PR cover.