Hi Andy
Think it's very wise of you to have considered this potential duplicate content problem.
Having a rel=canonical tag on the separate categories, or even a tag on them, would make sure that the URL is not indexed in Google, thus removing any potential duplicate content.
I can't really see a way of having both a main deals URL and a category deals URL both being indexed and ranking because, as you have said, the pages would either have zero content or duplicate content.
With that in mind, I think you're current format is the best one. Having a big /deals page with all your offers on it will hopefully provide lots of rich content so that people will link to the page, which in turn will rank the page for a number of keywords - while you're also allowing people to filter down and get to what they want. Just make sure that the separate sub-category pages have either enough unique content on them, or a canonical/meta noindex tag on them to avoid a duplicate content issue.
I would say though that as you're site aggregates daily deals, you are at a bit of a risk of still supplying duplicate content from other sites. I'm not sure how the deals are fed in, but if you get a rush of deals en masse from one website and the deal's titles and descriptions are all the same, this might also be seen as duplicate. If you can off-set this with unique content on the page and a system to put in your own titles/descriptions, then it shouldn't be a problem.
Good question this - would be interested to see some other Mozzer's POVs.