Jenny made some good points in her comment on the "answers to Matt's q" post that I suggest you check out. I want to take a moment to continue those streams.
I can't find the link, but someone a lot smarter than myself said recently (paraphrasing) that it's a sad fact of this country that people whose ancestors have lived in California since it was part of the Spanish Empire get treated like "immigrants" by people who are the descendents of Okies. I imagine that has mixed effects on people: some feel a connectedness with illegals and want to help them, others feel like they're suffering for someone else's actions and resent them for it.
Even that notwithstanding, though, Jenny's 100% correct that it's stupid to assume that Hispanics represent some singular voting block in the way that African Americans or Christian conservatives do, because there's not as much holding that group together. Catholicism in America is pretty evenly divided along political lines, and there's really no reason to suggest that non-immigrant Hispanics (i.e. those who have assimilated into "mainstream" American culture) would necessarily share any political affinity with immigrants. It's sorta like saying that everyone of Polish or German ancestry in the US should have the same political proclivities as Polish or German immigrants or even immigrants in the general sense.
That being said, it does appear that Hispanic immigrants (and judging from pictures of various protests, immigrants generally) share an affinity with, or at least sympathy for, illegals. As Jenny mentioned, the protests were not comprised solely of illegals, and in all likelihood were not even majority illegals. This is a potentially huge voting block (immigrants, that is) that gets made into a whipping boy by nativist conservatives every time some foreign power scares the bejesus out of them (for a little light reading on this point, just google "Operation Wetback" to see a particularly disgusting former manifestation of what we're seeing now).
It's a subtle strain of racism that conflates "Hispanics" and "immigrants," and that racism appears both in Congressman Hayworth's defense of the Arizona proposition, and in my own inability to immediately recognize it for what it was.
This is a conversation, and an admission, that needs to be made on a national level if we are ever to deal honestly and seriously about the immigration issue.