Does anyone know this is happening? I've tried deleting my interface folder and enabling/disabling all my mods.

http://i42.tinypic.com/eal2rl.jpg

Hmm, wierd. Have you tried having Steam verify the local cache for Skyrim? Seems like something got corrupted.

I don't remember exactly what it's called, but if you go to the Steam Library, right-click on Skyrim, choose properties, there's a button somewhere in the properties window which is something like "Verify local files" or something like that. If you click that, it should look to see if any of the game files are changed, and it should re-download any corrupted files.

Something's interfering with the values it's receiving.

The output wants to display a type of number and is receiving the wrong data type hence why it displays NaN(Not a Number). If you've altered any files recently or activated any mods, they might be interfering with it. Something may have gone wrong with the UI replace. Try reinstalling it?

Yeah I tried that - the odd thing is that you can see they have a weight in the SkyUI menu, but not when you 'inspect' them.

Yeah, it does this without SkyUI installed aswell.
I'm only using two mods, SkyUI and a House mod.

If you start a new character, do you have the same problem on the new character?

Just made a new character, same issue.

Have you tried dropping an item and seeing if it's weight is properly deducted from your total carried weight?

Yes, selling and buying items decreases/increases my carry weight, respectivly.

In my programming experience, I get NaN when I divide by zero. So my guess is you have a mod that reduces the weights for some/all items. If so this value is not being set, and coming up as undefined or zero, both of which give NaN. Start getting rid of mods thill you find it. (after trying the otehr suggestions)

As I understand it, SkyUI gets its values from SKSE. Might want to try reinstalling that, as you may have a corrupted file.

You might also check to see if there's a newer version of SkyUI or SKSE available than what you currently have installed.

Disabling both SKSE and SkyUI still causes NaN.
I'm going to attempt a full re-installation of everything (game and mods) and let you guys know how that goes.