I read through the license but it's too complicated for me to understand.
If you are purely coding for your own project, then you don't as your not giving it to anyone.
If your site is heavily integrated with YAF and makes use of its methods then the GPLs copyleft will cover your existing site code.
If its pretty much seperate and let say just uses the Forum control in one of its pages and makes use of teh same membership; it doesnt.
"It's a case of RTFM.. the only problem being we don't have a manual!"
When I post FP:Mek in a topic, I'm leaving my footprint there so I can track it once I get into coding/supporting. (Yes I stole this off Ederon 🙂 )