I don't think we have a guide on this and I'm writing it cause there's no explanation in game, and often when I mention this feature I get 'what? you can upload directly to youtube?'
Well yes, you can. Uncharted 3 features means to upload your multiplayer videos directly from Cinema mode, to youtube. Along with some other options that allow for exporting for editing, or uploading long matches.
Recording a Clip
Before you can upload a clip to youtube you need to make a recording. You can record an entire match, or just a short 30 second clip. Obviously depending on the length of your record, the length of your video render, and then later; upload will differ.
Once you've recorded a clip you want, carry on down to the next step.
Converting to video
Before you can do anything with the video outside of Uncharted 3, you need to convert it to video. In order to do this select a recorded clip (these are subheaded 'edited' in the local files menu), select them with X, then select 'convert to video'. The video will now be rendered, it plays the UC3 cinema file whilst it rendered. Note that Uncharted 3 records in 480p, if you want better quality you'll need to record with your own devices.
Uploading to youtube through Uncharted
If your clip isn't too long, you can upload it directly from the games client itself. Once your render has finished you'll be prompted with 3 options. 'Export to XMB', 'Upload to youtube' and 'Post on facebook' I think is the third option.
Export to XMB will take the video to your Playstation XMB. This is useful if you want to upload videos longer than the Uncharted 3 game itself allows, or want to move it around to another device (PC) for additional editing.
Upload to youtube will upload the video directly to youtube for you. If the video is short enough in length (I'm unsure exactly what the limitation is, I know videos over 5minutes can't be uploaded directly though. If it will allow you to, this is a simple process, it prompts you for a login and password for youtube, and will then upload the video to the defined youtube channel. Uncharted 3 uses heavy compression so the upload is usually very fast.
Uploading to youtube via the XMB
If your video is too long, then you won't be able to upload directly from Uncharted 3's cinema menu. However you can still upload it directly from your Playstation 3. Instead of 'upload to youtube' hopefully you selected 'export to XMB'. Now exit the game and go to your XMB's video menu.
From the video menu, you should see the video file you exported, it will be named whatever you named it when choosing to export. Now select 'video editor and uploader'. Then select, 'Create New Video' then file you want to edit and upload.
Once in the video editor it should be mentioned you can also use this to make minor edits, like text-based notes, and speed up the clip if required. This might be required if your clip is especially long, as youtube has a limitation of 15minutes for standard user uploads - thus you may need to speed the clip up.
Either way once you're happy with the video, and finished editing, select 'upload'. Agree to the terms and conditions and choose to upload it to either youtube, or facebook. Whichever you select you'll be prompted with a request for the pass/login of the account you want to upload it to. Once input, the upload will begin.
Once the upload is complete youtube spends a little time processing the video. But you should be able to view it on your channel within 5minutes of the upload from the XMB.
The XMB video editor uses less compression, and therefore takes longer to upload videos than the Uncharted 3 in-game uploader. However I think this means that they're a little better quality too, so that's something to bear in mind when considering how to upload your videos.
You can also export the videos over to other software, like After Effects, or Sony Vegas if you want to do some extra editing. This is done by selecting 'copy' from the video on the XMB, and copying it over to a USB storage device.
Quality Comparison
Here are a couple sample videos I've uploaded. One from the Uncharted 3 uploader, and one from XMB video editor. You can check these out to see the kind of quality you get from each method of upload.
Here's a video I uploaded with the in-game facilities
Here's a video I uploaded with the XMB video editor
Personally I think the XMB upload offers much better quality. But you can decide for yourself.
