Errors have and WILL always occur, its a fact of life so why not take the opportunity to own your own bad news and offer some guidance? Here are my tips on what you should and should not do with error pages.
Adrian's top 5 error tips
1. Always take the blame
Your error message should always put the blame squarely on your shoulders not the users. Its human instinct to feel like you have broken something and feel bad about it. Let them know its not their fault.
2. Be positive
Error messages written in a negative tone (or even neutral) can suggest that the user screwed up, or did not follow instructions.... BE POSITIVE.
3. Be casual and humours
Your writing should take a conversational tone and use some humer, self deprecation humor is best but if your uncomfortable about that then you dont like taking the blame do you?
4. What now?
Always tell the user whet they should do next. You can give them a link or a series of links, even a search box works well.
This is our latest error message for http://siiam.net which is a system that accountants use with their customers. We use humor in the image, take the blame with a positive tone and tell the user what to do next. 
Grooveshark example
These guys have nailed it. They have all 4 things covered... and the bear is so cute and a bit of an icon. You can even buy the t-shirt. Now thats owning your bad news! Good on you @Laura from Grooveshark. 
Super simple and humor that makes light of the error. This is all great but no instructions on what to do next? Apart from that I love it's simplicity. 
iStockPhoto Example
Covers off on everything but the humor, and not keen on making it the users responsibility to report the issue, we'll as a user I didnt know if I had to report anything or not? Not a good place for me to be as a user. 
Conclusion
I hope this helps you in creating your own error pages and if they do see your error page, they can move forward calmly while you fix things in the background. Please share your error pages or others that you come across. Cheers, Adrain As a followup on this I came across this great fail page suggestion for twitter. image shown above.
Some other Inspirational 404 error pages