They do declare copyright on the design, but that doesn't protect against designs that are merely similar.
For more interesting discussions about where to draw the line on imitating others, you may want to check out a little company called Zynga.
In fact, your pricing page(http://agilezen.com/pricing) is clearly inspired by basecamp. Absolutely nothing wrong with it(I don't think it is in ripoff category). But for someone to have such stringent standards when evaluating others, it's odd.
Can you elaborate? That is a pretty huge accusation.
The colors and font of the header are identical, including the use of a single green link. The fonts used in the headers don't match the logo or the buttons, and have altered kerning just like on Basecamp's site.
I honestly couldn't give a shit either way. The OP asked for feedback, and I figured it was better he heard it from me than from 37signals' lawyers.
It doesn't look like 37 signals has trademarked that particular color scheme or design, and I don't see any evidence that they own or have an exclusive license to the font in question; so there really seems to be no legal concern here at all.
Everyone uses other designs as inspiration (myself included), I'm just saying you should put some distance between yourself and the original inspiration before showing it to others.
The simplest would be to ask if it is ok. I don't think they'd have a problem with it.
The only similarity is the top header black bar.