It's a helper library for working with analog multiplexer / demultiplexers like 4051 - it's not massive but something I've put together because I use it a lot and am sick of copy/pasting code from one sketch to another to do the same thing over and over.
I would usually release using BSD but just wanted to make sure there was no quirkiness than would come as a result of that, given main arduino code I believe is licensed under GPL2
Your code and the Arduino code-base are (likely) completely separate; you didn't derive your code from anything in the Arduino code-base, did you?
Likely, the Arduino code is GPL2 because the code it is based upon (Processing and Wiring) is GPL'd. You could release your code with a BSD license, just as long as you know the possible consequences of doing so. For instance, I could take your code, make modifications to it, distribute it, but never release those mods back to the community (much like Apple did with OSX from BSD); I would still have to credit you, though.
I personally like the GPL for that reason; it keeps the software open and evolving, so that no one player can later scoop the whole bit up, make a bunch of mods, then close off their port and sell it, and make a ton of money off the backs of those who came before. Instead, they can still try to make their ton of money, but they must release their mods back to allow them (if they are judged worthy, of course) to be re-incorporated back into the base.
Now granted - both of those explanations of GPL and BSD licenses are very superficial, and don't touch on everything about either; just make sure you are aware and understand the ramifications long-term of releaseing under whatever license you feel is appropriate. Your library may not ever become someone's "cash cow", but then again, you never know in this world (and even if it doesn't it might become an integral part of someone's larger business - you just need to decide whether you or the community will care in the end; it really only becomes a problem if the end-product that is based on yours becomes more popular than the "community" version, with more features, and you have to pay for it, and you're not allowed to see inside it anymore - but your name will be mentioned, right?).
For many of these libraries, though - either BSD or GPL (or public domain, or a proprietary license) will be a-ok and not cause a conflict with the license of the development environment (provided, again, that you haven't based your code off of anything in that code-base, of course).