This one might come as a surprise to developers: If you use [MS] like Ribbon UI (Word 2007, Excel 2007) in your application you have to get a license from [MS] in order to legally use it. And it doesn't matter if you buy a ribbon component from 3rd party vendor ([DevEx] makes very nice one) - you still have to get a license from [MS].
Quote from the link above:
"To that end, Microsoft has created a royalty-free licensing program that will enable developers to build applications that have the look and feel of the new 2007 Office system applications. The new program will license elements of the new UI to software developers and component vendors on a royalty-free basis."
[MS] has done something similar if you wanted to play with Office native file format - you have to sign an agreement that you won't use that knowledge in a competiting product. Now they are extending it with requirement to follow their UI guideliness, too. So we. developers, might be constrained in extending Ribbon functionality.
I wonder how much is this a precedence. Imagine that you are building a complex application and you have to obtain licenses for every part of it that is similar to something somebody already done. At least it is free for now but this might change in the future...