I expect Bush to drop under 40% approval when Rasmussen reports his daily approval ratings on Tuesday. He has only dropped under 40 for two days (over the Easter weekend) in Rasmussen polling. This will be his true low point as Bush supporters may have been less likely to respond to polls over that weekend than those who do not support him. So those 39% approvals may have had a pool of respondents that was biased against Bush.
Regardless of your take on Rasmussen's biases, his daily polling gives a better estimate of movement than less frequent polling. Bush may look better on Rasmussen that in other places, but the slow drip, drip of support here as well as in other polls can't bode well for repubs.