According to a "trusted" source during GDC 2010, IGN AU reporting that Insomniac Games, creators of the Ratchet & Clank and Resistance franchises, is working on a multiplatform title for the PS3 and Xbox 360.
Although the developer has had long ties with Sony, the company doesn't own the studio, so there is nothing stopping Insomniac Games from developing games on other platform. Insomniac Games, which is based on Burbank, California, opened a new studio earlier this year in North Carolina, which may be related to this new news.
Last year, an alleged billboard advertising Resistance 3 was put up during the filming of a yet-to-be-released film entitled Battle: Los Angeles. Details can be found here at Resistance Union. Since Sony has published previous games in the series, it's unlikely that this news relates to the title. Instead, it's more likely that this multiplatform game will be an entirely new title.
Both Sony and Insomniac Games have yet to confirm the rumor.
Source: IGN AU