Since I got my N1, 6 months ago I got the same issue.
If I keep connected over wifi for a long period, when I turn it off my 3G won't connected automatically. I have to enable and disable the option to use only 2G connection several times to it connect again, if I didn't do that the connection won't get to work.
Any place I use wifi is the same thing, it never connect after i turn off the wifi.
I'm using the stock froyo 2.2.1. No root.

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