Saying we're at war with terrorism is like saying we are at war with bush fires, or car accidents. We can fight these things, we can look for the causes and try to prevent them from happening, we can raise people's awareness - but at the end of the day you cannot declare war on an idea.
This whole 'war on terrorism' thing is a buzz phrase to get people whipped up into a frenzy and to allow governments a bit of 'flexibility' in how they handle things. I'm not necessarily saying that I disagree with the measures that are being taken but technically, you can only declare war on a country or a state, not on a concept.
If you wipe out one terrorist organisation, another one will spring up in it's place. It's not like WWI-WWII where there were clear aggressors and defined battle fronts. You can't say to 'terrorism' "if you do not withdraw from such and such a country, we will have no alternative but to declare war on you". It's not a war that can be won.
The declaration of war allows you to do certain things and take certain steps that would not normally be allowed - but you are also bound by the Geneva convention. The reason for declaring war on terrorism was to open the way for the US to send troops to another country. It had to be done or their constitution would not have allowed that to take place. If it ever went to court, I doubt that they would have a case.
The conflict that the current focus is on has been going on for hundreds of years. It's not the only struggle that attracts terrorism but because it has hit at the US, Europe and now England, it's profile has been raised to predominance. Who knows what will happen in the end. Maybe there wont be one.