Bought iPhone 3GS with iOS 5.1, can downgrade to 4.1, any hope of going to 4.2 or 4.3? I'm an iOS developer and have been searching for devices running older versions of iOS to use for compatibility testing.  (the iPod touch I have been using for this purpose recently gave up the ghost)  You wouldn't believe how hard this has been, even on the used market -- either sellers don't specify the iOS version in their item descriptions (meaning you have to take your chances), or they "helpfully" upgrade to the latest iOS before selling it.  Anyhow, I thought I finally hit pay dirt when I found an iPhone 3GS whose auction prominently listed iOS 4.x as a selling point.  Well either the guy was outright lying or he didn't know what the hell he was talking about, because when I received the device, it had iOS 5.x on it.  I know I could probably file a Item not as described claim or something to that effect, but I am not in the mood for a long drawn out battle, hostilities, negative feedback war, etc. and so at this point I was ready to cut my losses and live with it running 5.x.

That is, until this afternoon, when I was idly playing with TinyUmbrella, and found, to my delight, that Cydia has a saved SHSH blob for this device running iOS 4.1.  Apparently this device must have been jailbroken at some point in its lifetime.  So I was able to restore to iOS 4.1, and jail broke using limera1n.  All is well.

Now for my question: Is there some way I can get this device to 4.2 or 4.3?  (4.1 is a bit too old for my taste)  My (admittedly very minuscule) understanding of SHSH and related matters leads me to believe that the answer is "no," but I'm asking anyway in the vain hope that there is some hacking trick or something that would make this possible.
I'm surprised that you found an SHSH blob for a firmware that low. Consider that a treasure. Unfortunately, though, you can only install the version of iOS that corresponds to the SHSH blob. The blob contains a signature that is unique to the device and unique to the iOS version. It is like the key to your device for that version. It can't be faked, but it can be saved. So in short, you're stuck with iOS 4.1 or 5.1.1, and there isn't anything that can be done now to change that (besides, the change from iOS 4.1 to 4.3 only added AirPlay/Print, Safari's Nitro JavaScript engine, iTunes Sharing, and Ping; which shouldn't be big enough features to worry about unless your app integrates AirPlay).