Try simply power cycling the router, sometimes that's all it takes. Then power cycle your phone if that doesn't work. If none of that works try removing all encryption to see if it will connect, if not then the router could have some compatibility issues with the droid (or the droid could have a hardware issue). In that case you can try updating the firmware on the router (usually available on the manufacturer's website).
If having no encryption causes the droid to connect then you can try the different types till you find one that works or you can narrow it down to the ones that don't. It will obviously require some troubleshooting to nail down the issue but you shouldn't jump to conclusions before you try the droid with other networks as sometimes the problem is the router even though it works with other devices.
So far I've had to no problems with my droid. At home and work it connects fine and both are wpa based networks. I've also used it on several of our clients' networks without incident. Others have had bad luck but because there are so many different routers it's hard to come to a simple conclusion. It could very well be a problem with the droid but it would only have to be on some units.