How should we rate the difficulty of climbs on the website?

When someone adds/edits a climb, they should be able to select a difficulty from a list. This means that people can search for climbs based on a difficulty.

For example I'm climbing at around 5.9, so I can search for all climbs rated 5.9 within 1 mile of zipcode 10031.

So - what rating system should we use?

The primary focus of the climbing section at the moment is bouldering, so it would make sense to use that. However people will be adding trad routes too, and may be more comfortable grading based on that.

My thought - the list should contain both, eg:

5.5
5.6
5.7
5.8
5.9
5.10a | V0
5.10b
5.10c | V1
5.10d
5.11a | V2
5.11b
5.11c | V3
5.11d
5.12a | V4
5.12b
5.12c | V5
5.12d | V6
... etc...
5.15a | V15

This list is taken from Wikipedia, so I've no idea about how valid it is!

Or is this a really bad idea? Are bouldering and climbing grades completely incompatible? Should bouldering routes be tagged with bouldering grades, and trad routes tagged with trad grades?

Comments please, and feel free to add links to useful web pages!