I don't need ideas on where to travel. Recommendations on destination come from family and friends ... the game is then to fit it into budget / vacation days / schedule, etc.
Though it hasn't got there yet, the plan with the main Adioso site is to do what you described; if you have a destination, budget and vague timeframe in mind, Adioso will monitor options and prices, and alert you when a deal becomes available.
Also I suspect the minimum price point ($500/£250) is too high. The text of the site is challenging the user to spend more ("How much do you <3 me?") so it will be more difficult than usual for users to select the minimum (or in their words: "A little").
They'd do better to make the submit button actually look like a button, and to set the minimum to $0 (even though no flights will ever be found for that price).
I think in the problem domain the Adioso guys are in, this is a fair trade off. Not all first time users would understand the concept of natural language search and would instead be looking for "from and to" dates.
What's you suggestion to improve the main site? I think it's pretty good.
I have no thoughts on the interface per se, I just think that if you have to admit that your interface is confusing, you've already lost.