Ok, lets take a look around the web and see how people are doing tabs in apps these days.
You are going to see a lot of similarity. You'll see that even the CSS Tab tutorial (2nd search result via Google) bears a resemblance to 37signals own tabs - just change the colors a bit. Beast's links may look a bit out of place... but since that's one of my apps that's been mentioned in "rip off" circles I thought I would include it in this comparison.
CSS Tab Tutorial
The 2nd result in a google search for "tabs css": Updated Simple CSS Tabs | silverorange labs
Example found here as they have changed their site design.
Working With Rails
Tabs are all about the same everywhere.
CSS tabs on a solid color background... Design Piracy? NO. Wait, there is a difference though...
Square vs Rounded tabs
I think it's a less is more thing. I personally find the square look cleaner. And of course it's just good usability that the tab's background color flows into the content. This makes it as more like a real life tab and not just disconnected navigation.
37signals gets that right and we all do likewise (most of the examples do). Beast doesn't bother because really those are navigation links, not tabs.
Jason speaking at SEED
So it looks like 37signals doesn't have a corner on the CSS tabs industry... but maybe Jason doesn't mind the tabs...
A friend reminded me that Jason actually addressed the issue of tabs in an open discussion panel at SEED this year in Chicago. Evidentally similar tabs didn't bug him too much.
My friend: he [Jason Fried] laughed and said 'no' and like 'how could you really do them differently?'
I'm glad we agree.