In competition with GHI's FEZ Gadgeteer based platform is the Netduino Go. I don't know what the timelines are but I believe this was produced as a more versatile competitor to Gadgeteer by Secret Labs LLC. The boards are similar in that they use module port connectors, but in the case of GoBus, all of the ports can be interfaced to any module, so it makes the ports more universal rather then semi-dedicated as in the Gadgeteer.
"So we created a compatibility mode for Netduino Go which lets you use most S-U-X Gadgeteer modules with Netduino Go." (interesting arrangement of module letters). This could be a big plus in the future.
This platform appears to have all of the advantages of the Gadgeteer platform and perhaps a few more because it uses a standard GoBus architecture. This means there are no specific sets of ports that need to be used. On the down side to that is the need to decode the bus commands so each module would require additional circuitry.
I haven't directly compared the prices of modules between Netduino Go and Gadgeteer, but I would expect them to be in about the same price range, making this a more expensive hobbyist prototype platform than say an Arduino Uno. Although the Netduino Go also supports the .NET Micro Framework and C#, I believe it would be difficult to port the code to a smaller commercial platform.
If you are considering a prototype platform with a lot of expansion capabilities this might be as good a solution as Gadgeteer. Because of the bus, it may have better long term interfacing opportunities. Again, if the idea is to shrink your prototype down without a complete rewire/re-hardware, which is my case, this might not be a good option. If on the other hand you want to build a one off, or expensive production models, this would be a good platform.