Producer Shuntaro Tanaka told Famitsu (via 1UP) that VC2 is being developed for the PSP instead, in order "to allow a broader spectrum of users to discover and enjoy what makes Valkyria special." Tanaka added that the series could return to consoles in the future, though.
Despite critical acclaim, the first Valkyria did not sell exceptionally well. The PSP has a user base of about 50 million (roughly twice that of the PS3) so perhaps the change of platform will help the sequel move a lot of units. I assume Sega's dream scenario is that they'll do so well with VC2 that they can release a third game for PSP and PS3 someday.