So, how does this work? Will everything be fine if they maintained the same look and feel and intentionally obfuscated/added more junk to their code to make the code look different?
But that's just how it is in the design industry -- new designs come out quickly, so it's usually not worth the effort to fight knockoffs.
That's because design is an area covered by trademark, not copyright. You can trademark a design if it's distinctive or innovative enough, at least in the U.S., and have protection from imitation. You don't even have to register a trademark to establish rights, you can declare your rights simply by using the (TM) symbol, although you do have to register in order to defend the rights in federal court or claim international protection.
http://www.uspto.gov/trademarks/basics/register.jsp
From a whois check on their domain name, however, Plurk Inc. appears to be Canadian. Anyone have details on the what differences there are in the Canadian trademark system?
But simply cloning a user interface is legal, at least in US case law.
Can you give us citation that it is legal, or are making your claim based on the above lawsuits?
Having a look and feel that is "similar" but "not the same" is open to debate, but having 90% of the interface ripped off, pixel by pixel can't be just "ok". We don't know that because something so dumb didn't happen before in the highest level.
>None of the source code or machine code that generated the menus was copied, but the names of the commands and the organization of those commands into a hierarchy were virtually identical.
From: http://en.wikipedia.org/wiki/Lotus_Dev._Corp._v._Borland_Int...
In this case MS not only copied the interface of plurk, but from the look of it they also stole the code. Such a blatant copyright infringement has never happened before at this level to the best of my knowledge.
Your claim that it is "legal to copy UI in US law", based on the above example, is false.
Copying a menu, its command name and hierarchy is not the same as copying an interface. A menu is only small sub-set of user interface.