In case a single cryptic twitter post isn't clear enough: SBCL got accepted as a mentoring organization to Google Summer of Code for 2013. Our hastily-constructed organization profile page is available, and anyone who is interested in mentoring students on SBCL-related projects (and has a few hours to spare a week over the summer months) is welcome to sign up to the site and request mentor status.
We also have an indicative list of potential projects, mostly constructed with extreme speed just before the application deadline by Paul Khuong. I say “indicative” because I think the most important factor in successful student projects is intrinsic motivation, which must (of course) come from the student themselves. So I'd suggest that any potential student applicant should use those ideas as a guide, rather than a finished application: feel free to mix and match goals.
I think it's also worth emphasizing that it's unlikely that SBCL will be allocated more than a handful of student slots: the guidelines do say that organizations new to the programme are likely to be given relatively few to see how they cope; although I've mentored for the program before it's not clear whether that experience, with LispNYC, will carry forward into this new organizational setup. So students keen to work on SBCL would be well advised to engage early, and demonstrate that they can at least begin to work with the codebase (a trivial typo-fix patch, with correctly-formatted commit message, would be a great start, for example). It's also worth saying that Google, not the SBCL organization, has the last word on which students are accepted.
(Thanks to the Common Lisp Foundation for agreeing at very short notice to handle the financial-related side of SBCL's participation.)