Apple has announced WWDC 2010 for 7th June. WWDC is when Apple traditionally announced the new iPhone. As to when it will be released after that point, it is hard to say - could be a week, could be a month, but probably no more than 6 weeks as an maximum.
As to the features, the lost prototype that has done the rounds recently boasts a higher resolution screen, front and rear facing cameras, a flash for the rear camera, and a spiffy case redesign, and will probably also feature a faster processor (likely based around the Apple A4 in the iPad), and maybe improved graphics.
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Whatever happens, I'd recommend waiting. It's only going to be a month until we know the actual release date and pricing. When the new phone is out you can decide if the new one is for you; if not you'll likely be able to pick up the 3G s for a lower price.