As far as I know, the firmware of both is identical, but you will have to pay for major firmware updates for the iPod Touch, at least if aren´t a developer as if you are, you can at least download beta versions, I am not sure about the release versions though. I also think that the hardware of the latest iPod touch is a bit better than the hardware of the iPhone 3GS.
If I understand it right, the iPad also uses the iPhone OS, so that you can just get an iPod Touch and develop for iPod Touch, iPhone and iPad. But as for example the screen dimensions of the iPad are different as it is 4:3 what the iPhone and iPod Touch isn´t, you should may get an iPad as well if you somewhen decide that it is worth it. But I think for the start the iPod Touch is a good choice, even though I like my iPhone quite a bit