So, as you can probably guess, I downloaded Carbide for the purposes of creating a theme to use for my Nokia phone (N95 8GB, if anyone's interested). Managed to put all the elements together, but I've run into a rather irritating snag: How the (insert preferred profanity here) do I turn the finished product into something my phone will accept (.sis)?

yes once you have ur theme made it you have to create a package which will convert it to a sis file which can then be put your phone

+1 for what Anuj said.
Here are the exact steps though:

Tools > Created a New Package

A window will pop up.

Click Next.

You will need to sign your theme. Click on the "Modify.." button.
Create new key pairs.

You need both the key and cert file (Carbide can generate one for you).
No password is required.

Click finish.. then Carbide will start compiling the theme for you.

And.. you're done!