The Mytouch has a GPU in the system, the Ally has none. If I remember reading right, that was one of the problems, the phone uses the main system ram for graphics processing, which the mod over at LG Communities mentioned was a problem. A phone closer to the specs of the Ally is the G1, and it's definitely NOT getting Froyo, as it's hardware is too limited to run it.
They haven't definitely said, NO FROYO, but if the system just isn't powerful enough to handle the OS, or it not be a step up from what you have now, why would they do it? For what it is, the phone works well, and hopefully the LG engineers can squeeze Froyo in there, but a petition demanding it won't really sway their decision.
If getting Froyo is that important, maybe folks should look into the Droid2, it's got the hardware keyboard also, and is supposed to have Froyo pre-installed at release. Couple that with a 1ghz proc, and a total of 16g memory by default (8g built in, 8g SD included), it'd be a win/win.