It’s true, I don’t have an account with Last.fm. I have a vague idea what it’s about, but I’m afraid it’s not interesting to me. A lot of things aren’t, though.
shell.fm is in AUR as shell-fm, and there is a shell-fm-git, but it looks like shell.fm hasn’t seen updates in quite a while, so there might be no tangible difference. I only found shell-fm in Debian.
Sorry to be so uninformative. If you’ve worked with shell.fm and can offer more than I, please feel free to make suggestions. It looks intriguing, but aside from what you see above, I can give you no guidance. 😦