Try a (temporary) layer of plastic wrap around the sides to cut out drafts and see if it goes away for you also ..

What feed rate are these axis running? What is ideal?

Thank for any help!

Again, this has been annoying me too.

If, jetguy or nophead, or tbuser, or Ed, or Dave D., or John A., or …. Other old timers who may know and read this… ????

Good solder job.

Test without installing. Prop your board safely connected to the bot and away from flammables and do not let it short on anything. Note: your DC board looks fine. This is just a common sense quality control practice you will do. Mostly because its a hassle to install and find your solder job stinks or something else goes phhhtttt. Again, in the photo yours looks quite good.

Turn on the heater through the control panel to ???? maybe ? 45C. Wait and watch. Ramp the temp up a little at a time. Let it stabilize and observe. Take ten to twenty minutes with your test? Maybe burn in an hour? Not needed but, for your comfort level. It is important to have confidence and to be safe.

You could have just scraped the board trace and soldered the pin back on. But, your fix looks good. Only one thing. I do not know if the heat on the backside will get warm enough to melt your insolation. Another but here - But, there is limited shorting potential where your ground wire is so you are likely - Good to Go.

If it did explode - would it be confined to this board, or could it take out the motherboard/arduino/extruder controller?

http://en.wikipedia.org/wiki/MakerBot_Industries

Reprap, while nice, is a pain to source all the hardware (threaded rods, nuts bolts, platic printed pieces, etc..) and then a real pain to build and adjust. The idea behind it was common components you can get anywhere in the world thus it is a very neat project from that perspective. Makerbot saw this and said-let's take the hard parts out of the design, make a laser cut frame that fits together with T-slot like Ikea furniture and simplify building it for the novice user. That is the key element of the design and again, was based on decisions of selling a kit, ease of assembly, and lowest cost to manufacture. In other words, a prime business model (make something at a lower cost to produce, that requires less labor and effort).

Sellers of Reprap kits have a problem in that the hardware they sell has a fixed cost making up most of the price of the product. The only "value added" is that they source the components, cut parts to length, and provide some instructions. They can only charge so much of a "premium" over the cost of the hardware. Makerbot saw this and said-why do we do it this way? The original Reprap design has lots of flaws and tradeoffs due to the fact it was designed to be "printed". The truth is, it's not a great requirement. Some people just want to buy a kit and be done. This is especially true if the kit is easier to assemble, and further, lower cost. Makerbot managed to achieve this as Cupcakes did sell for less than the typical Reprap kit. As I remember it, Cupcake basic was around $650 and the typical complete Reprap kits were and still are $700 -900.

First, Cupcake is a derivative of the Reprap project.
http://reprap.org/wiki/RepRap
Thus Reprap is the "original" open source 3D printer and as such, the open source license allows for commercial use as long as the new design published the source files and information-thus Cupcake and MakerBot follow that license to the fullest extent (answering questions 1 and 2).
Gen 3 electronics, gen3 firmware and replicator-G (the open source control program) are what drives this project. Without the open source free software, the hardware would do nothing. http://replicat.org/
http://reprap.org/wiki/Generation_3_Electronics
http://reprap.org/wiki/Generation_3_firmware

So to recap, you really need to research Reprap since that is the basis of Cupcake. Makerbot saw the chance to use open source licensing of a commercial version to sell kits. Yes, they added some design but note it was mostly in house (in other words, the design was not built in the community with roles-question 5). Thus, they are an example of how open source licensing works in the business world. They publish the designs to each of the machines in accordance with the license agreement as well as the use of the open source software. This is because without licensed use of the software/firmware-they would have to start from scratch. Thus technically, replicator-g is an open source piece (although they have some big part of it as well).

6. Which platforms are used by the community ( Wiki, Forum, … )
Pretty much the Wiki and Thingiverse
7. How do you get your project known? (advertising)
Makerbot went on a extensive add campaign last year after they got some capitol (money) invested. They were in every tech magazine, and several popular TV shows.
8. Do you reduce the costs by using open source?
Basically, they did not have to develop the entire system. They depend on the open source software and firmware. They did design the hardware in house to be mass produced on the cheapest materials (plywood) and sold as a flat pack kit. Key part of the design is access to a laser cutter since nearly every part if it is designed to laser cut.
9. Why did you just open the project after developing the CupCake and not earlier ?
It was open source-the Reprap project. They used open source code in the design and thus HAD to publish the final design in accordance with the original license.