I think a new phone will be released this summer. If Apple wants to continue to be competitive in the mobile market and keep the edge they have with the iPhone then Apple will need to release a new model in the summer.
My thinking about purchasing or waiting. It's silly to waste time thinking about the past or future. It's better to think about the hear and now. We have no control over what happens in the future. the past is what it is. You can't change that either. You only have control over what you do now.
In the summer(or around that time frame) Apple will release a new model. It could just be a minor upgrade. Higher capacity slightly faster processor, better camera, software update that includes copy/paste, push notifications etc.
Or it could be the revolutionary device that everyone here wants. Dual processor, discrete 3G graphics, higher storage, front and back camera etc.
If Apple releases a slight upgrade people who buy the iphone now won't feel so bad. They've been using their iphone for months and putting it to good use. Whatever minor software updates introduced will be available to 3G users as well.
If Apple releases a major upgrade, I wouldn't feel so compelled to get one right away. This is new technology and a new development path for Apple. Knowing from past experience their may be some software/hardware glitches on release. I'd rather wait for Apple to clean up the little bits and pieces and purchase the new iPhone around late december or this time next year. Whatever software updates should trickle down to 3G users as well.
If you can wait, then by all means wait. If you are struggling on a decision now, it means you really want want one. But make your decision based on needs and on your financial ability to afford the phone/contract. If you don't see yourself being able to purchase a new phone in 1-2 years then wait for the newer model.