Pika Scheme is a new implementation created by Tom Lord of Arch? fame (Arch? is a version control system). Currently, it is a work-in-progress -- not yet a useful implementation.
See Tom's homepage. But most of the pika links are broken/unimplemented placeholders.
There are also prereleases, a mailing list, and a project on Savannah?. The code can be found at these Arch archive coordinates:
lord@emf.net--pika-2004
http://arch.quackerhead.com/~lord/archives/lord@emf.net--pika-2004
Tom's homepage says "I am cautiously optimistic that it will be a complete implementation this year, hopefully by the end of the summer". It is not clear whether this was written in 2004 or 2005.
It seems that Pika Scheme sounds a great deal like vaporware. it is already 2005 and no new releases are out.
It's not completly vaporware, the C runtime is already more or-less complete, all functions from R5RS that don't require procedural arguments are implemented (in C). It is true, however, that Pika is currently stalled, presumably due to time constraints on Tom Lord's side.
Judging from http://web.archive.org/web/20040401134112/regexps.srparish.net/www/, it's at least been added after 2004-02-13.