My 3G definitely has some sluggishness on iOS 4.0 in some areas, some of the time. Some things seemed to be a bit quicker too.

Some people have reported that it is faster after a clean setup rather than a restore. Not that that is very useful, if you actually have data and settings that you want to keep frown

I've just mailed mine off to a recycling company, which I guess means I'm keeping my iPhone 4 for now.
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Remind me to change my signature to something more interesting someday