I don't recommend that people do this without a high risk tolerance, but I've talked to dozens of immigration officers without proof of onward travel (I managed to fill all the regular stamp pages on my last passport). Probably half of them didn't even ask, and most of the rest of them immediately accepted that I was going to buy some ticket out in a week or two. Maybe I got lucky, but I think "American software developer" just doesn't look like a real high risk to them, and I probably don't present myself as a backpacker who might run out of money while I'm there. I have brought printouts of savings and retirement account balances and such, but I've never had to show them. I would have been willing to buy a ticket out on the spot if necessary as well.

Weirdly, it's the land crossing to Canada that has always been most difficult for me. Once, after getting pulled aside to chat with an officer for 15 minutes or so, he eventually said, "so what you're telling me is that you're rich enough that you're not coming here to work in the oil field?" "Yes, exactly."