The image merely shows a Half-Life logo and the words: 'Freeman, June 14th'. The date coincides with Valve's would-be Portal 2 E3 conference. A Valve insignia sits at the bottom.
This could mean one of three things, for our money: (i) Valve will announce Half-Life 3 on June 14 at E3; (ii) Valve will announce Half-Life 2: Episode 3 on June 14 at E3; (iii) Someone in a bedroom has Photoshop and too much time on their hands.
Meanwhile, Ripten reports that a '360 and PS3 journalist' has been told by Valve that the new non-Portal 2 'surprise' at E3 won't be of great interest to them. A PC exclusive makes Episode 3 the more likely candidate.
Of course, this is all scurrilous rumour and conjecture for now. Whatever Valve's showing off at E3, we'll be there in force - ready to let you know exactly what's going down.
Meanwhile, we'll check in with Valve's team to see what they say about the new image...