Why exactly do you want it to work on the old Android version? Perhaps you could update Android instead, which may anyway be the better way to go. Which type of phone is it? Have you checked
http://cyanogenmod.com/ and other Android branches for support of this particular type of phone?
Given that this problem will disappear in very few years on its own, I have some understanding for not wanting to put extra effort into it.
The problem will not disappear on its own... a phone doesn't just update itself and there are actually people that don't buy a new one every year or two.
There's actually a cyanogen mod for my phone now (there hasn't been last summer when I last checked) and I'm going through the excruciating hoops to get it onto my device (backup, find old version of rom manager app (since newest doesn't run on android 2.1, whatever else needs to be done). That's not for everyone and his mom as you probably know. It's also dangerous (you can brick your phone). In addition to that, the process voids the warranty of the phone.
If BitcoinSpinner can be made compatible by linking against some other android.jar then this is a little work on the devs side (1 person) and _no_ work for many users, also the ones that don't even know what version of what? android? what is that? they have on their phone. Bitcoin should be not just for us geeks, you know.
Also think of the secondary phone market (like Africa, for example) that depends on used phones and therefore lags a couple of years technology-wise. Africa is a good example because bitcoin might find fertile grounds there.
I agree, if the effort necessary is high, it might not make sense. But you make it sound like I could just "install cyanogen" and everything is fine. Android 2.3 might even have higher resource demand than my phone can actually deliver.
For all those android <2.2 users: I tried electrum4a (based on python, should also work on 1.6), which works just fine. There's no .apk yet, so you have to go through some hoops to install it, ah, but hell, you can also just wait a few years, maybe someone makes an installer and the problem goes away.