Can you fix a bent pin in an iPhone dock?

Typically, No. Once a pin has been bent, it will never hold down to the dock. In an emergency, you can attempt to flatten it back out to the slot it belongs inside the plastic, but it will likely break. To do so, use a needle or pin, anything larger will likely cause more damage.

For a 2G iPhone, it's a much more complicated repair, and usually incurs more of a cost to fix than the iPhone is worth. However, you can ask for a custom repair.

For a more complete fix for a 3G or 3GS, you should get your bottom dock replaced.
iPhone 3G Bottom Dock
iPhone 3GS Bottom Dock

For an iPhone 4, you will need a dock replacement, ask for a custom repair.