I saw one today. I was going to report it, but I checked the rules first. Referral links is not mentioned in signature rules:
# Signatures:
Must be setup in your profile, and not manually added to your messages.
May not contain any pricing, plan, sales, etc. details. "Free" is considered a price in the context of this rule.
May contain a maximum of two smilies. Or one animated smiley.
May include two clickable links.
May include two colors, plus black.
Maximum font size cannot be larger than normal.
Must be kept to a maximum of four lines when viewed at 1024x768 resolution using the horizontal postbit.
May not contain links to other threads or posts.
Please keep special characters to a minimum.
Your signature is your signature. It is not for sale or rent.
Any signature that is offensive or insulting to WHT, its members, or its staff, is prohibited.
We reserve the right to ask you to change and/or remove your signature at any time, for any reason.

So is it OK to have referral links in signatures?

You may not post commission, referral or affiliate links anywhere on the forum. You may not link to sites whose main income is derived from affiliate or referral links.
*Anywhere* means also sigs and homepage :)

Where's it at? I'll get it for you. :D

I see that it is already taken care of ^^

That mouse is quick....:D

That mouse is quick....:D
Yeah, quicker than a bear huh? :D

Runs rings around me most days. Bears aren't known for speed (physical or mental).:S