Lamar Odom -- Response to Khloe Kardashian's Ultimatum ... Radio Silence

Khloe Kardashian's ultimatum to Lamar Odom -- get into rehab or get out of our home -- hasn't exactly forced him to take action ... in fact, Lamar's gone completely off the grid with his wife ... TMZ has learned.

We broke the story ... Khloe dropped the tough love on Lamar over the weekend. In the wake of his DUI arrest on Friday, sources tell us she made it clear to Lamar ... he needed to get help, or else the marriage is over.

But we're told since they spoke for 4 hours on Saturday ... Khloe's heard absolutely nothing from Lamar.

As one source put it ... right now Khloe is "totally in the dark" about Lamar's comings and goings -- and the prospects of saving their marriage are getting dimmer everyday.

But Khloe is refusing to let the drama with Lamar affect her social life ... and this weekend, she hit up a Labor Day BBQ fiesta at Katy Perry's house.

During the party, Khloe posed for sexy photos with her friends ... and later tweeted one of the shots to her Twitter page.

Just a few days before the party, Khloe tweeted (and later deleted) the words, "Smile and no one will notice” ... and after this weekend, it seems she's following her own advice.