Different Strings led me to this CNN/USA Today poll  conducted by Gallup. 

Here's a snap-shot of the data:

Was it worth going to war in Iraq?  April, 73% yes; June, 56% yes

How are this going in Iraq?
May 5-7: well, 86%; badly, 13%
May 30: well, 70%; badly, 29%
June 27: well, 56%; badly 42%

Would it matter to you if Bush did mislead [the] public on Iraqi weapons?
Great deal=53%; Moderate amount=22%; Not much=11%; Not at all=11%

Are you confident that the U.S. will find weapons of mass destruction?
March: very confident, 52%; somewhat confident, 32%; not confident, 15%
June:  very confident, 22%; somewhat confident, 31%; not confident, 45%

Legalese: the poll reportedly has a sampling error of +/-3% pts.  I present the information here under the "fair use" doctrine of the copyright law.

Now then, with that out of the way: the trend I see is a decrease in numbers that would seem favorable to the conflict.  While the numbers which might be construed as opposing the conflict hover around 50%, I take this as a hopeful sign.

Note to Donald Rumsfeld: whether the Iraqi conflict fits your definition of a "guerrilla war" or not, I think it fair to say a conflict resembles Vietnam when there is no longer public support for said conflict.  The fact this is being reported by CNN and USA Today - both relatively conservative outlets - emphasizes the decreasing support.