AFAIK, no. It's the same reason why we cannot use a Verizon phone on Sprint. Technically everything is compatible (in terms of the CDMA network) but the companies don't want to sign "foreign" ESNs.
I should also add that Sprint will not let you pay your ETF until you are locked into contract (your return period has expired). With Sprint you would essentially pay your first month's fees (around $85 after taxes) PLUS the ETF. The $100 you were planning on saving would be closer to $15. Not worth the hassle IMO, even if it did work.
There has been a lot of discussion on the topic, and a lot of people feel that buying an unlocked GSM model and then going the $45/mo straight talk route (uses AT&T's towers) is the way to go. Better coverage, faster data, with only a slightly higher monthly rate. The resale value of the phone if/when you are wanting to sell it will likely more than make up the extra $5-10 you spend a month on the plan.
Thanks again lordofthereef. What you say makes sense. I might consider Straight Talk, the only thing is I've heard if you want to have the option to tether, that is not supported on Straight Talk. Maybe that is a misconception though?