You've begged for them, and Sony's finally listened. Sony shot us an e-mail just now stating that "first and third party demos will be offered on an ongoing basis" through their official website. As we reported weeks ago, Killzone is now available for download, but Syphon Filter and SOCOM should be available in the coming weeks.
While these games certainly aren't the newest releases for the system, this is a step in the right direction. Hopefully, it won't be long until we see downloadable demos of Ratchet & Clank and third party games.
Demos at PSP Fanboy