JK Studios' creative and managing director James Kearsley has Little Big Planet 3 listed on his resume. Although the the details have since been pulled, a cached version of the document can be viewed here (spotted by Gematsu.com on Neogaf).
The resume points to the involvement of Sumo Digital, who have done bits and pieces for Sony in the past. Little Big Planet 3 was rumoured to be in development earlier this year, and it's looking increasingly like these rumours were indeed correct, and that Sackboy will be making a return in the future. Perhaps Sony are preparing to release information during this week's E3?