Can you elaborate? I'm the designer. While I definitely drew some inspiration the way that Basecamp is laid out, I don't really see any similarity in terms of the styling.

metalab styling (colors, font) maybe different but the layout and the flow of the application is very similar.

That's because they share the same graphic designer.. look at the portfolio.

Uh, 37signals use to be a design firm while they were building their startup. They design inhouse, only some sort of companion app for 37signals was designed by that graphic designer.