PIC: Gossip Girl's Leighton Meester, Penn Badgley Kiss on Set!

Credit: Aby Baker/Getty Images

Guess hell has frozen over.

Blair Waldorf (Leighton Meester) and Dan Humphrey (Penn Badgley) struck up an unexpectedly flirty friendship on season four of Gossip Girl -- but Brooklynite Humphrey's growing romantic feelings for Blair were thwarted by both her undying love for Chuck Bass (Ed Westwick) and an engagement to Monaco royal Prince Louis (Hugo Becker).

But, shooting scenes for the CW hit in NYC on Monday, Meester (in a dramatic pink gown) and Humphrey were photographed in a passionate kiss.

On the landmark 100th episode which aired Jan. 30, Dan provided the getaway car for Blair after her disastrous wedding to Prince Louis (who angrily vowed they'd have a loveless, obligatory marriage after she was discovered speaking romantically with Chuck).

No word yet on whether the lip-lock -- the unlikely pair kissed fleetingly last season -- was perhaps part of a dream sequence or the real deal.

Tell Us: Should Dan and Blair get together, already?