I wasn't on the Dev team for SpellComp (Michael Donais, Steve Schubert, and Rob Watkins handled it), so my knowledge only goes so far. Thus, I'm not a good source for in-depth Q&A; even if I were, that sort of thing should really wait for the website news & such later this fall.
I can say for certain that PH spells are NOT reprinted in it. It's one thing to collect material from dozens of sources in the aim of providing a "one-stop-shop" for spellcasters, but reprinting 120+ pages from a book the game already expects you to own didn't seem like a very wise use of resources.
The book doesn't have a "focus" other than "spells." Yes, there are some spells from FR products in there, but honestly the vast majority of "FR" spells are largely indistinguishable from "D&D" spells so my guess is most readers won't even register a difference. Sure, you and I might be able to remember the origin of a particular spell as coming from one of those spellbooks that Ed Greenwood wrote about in Dragon the 80s, but we must accept that we're on the fringe of that particular bell curve.
And speaking of that august periodical, I'll also confirm that Dragon magazine (from debut of 3E to present) was one of the sources for Spell Compendium.
Oh, and before everyone gets excited about making requests, remember that by the time a catalog appears, the book's already been designed and developed--as intelligent and well-meant as your requests and comments might be, we don't really have any ability to act on them. Like it or not, that's just one of those realities of our publishing schedule...
As for the "Will there be a Spell Compendium II" question, y'all know that we don't talk about products that aren't yet in a catalog. As is always true, sales numbers speak very loudly, so logic tells me that if this product does well we'd certainly consider a "sequel" that collected spells from additional resources, but I don't have any more insight than that.