This is an unresolved issue in the law and hence perfect for a moot court competition. And, in fact, it is an issue in the Robert F. Wagner National Labor and Employment Law Moot Court Competition. Forty-six teams of law school students (I would guess mostly 3Ls) have submitted twenty-three briefs for each side (employer and employee). There's a second issue regarding immigration status and remedies following the Supreme Court's decision in Hoffman Plastic Compounds, Inc. v. NLRB.
Back to the Gross issue, the Sixth Circuit just issued an opinion in an ADEA case earlier this month stating:
"Thus, when a termination arises as part of a work force reduction, the fourth element of the McDonell Douglas test is modified to require the plaintiff to provide 'additional direct, circumstantial, or statistical evidence tending to indicate that the employer singled out the plaintiff for discharge for impermissible reasons.'"Schoonmaker v. Spartan Graphics Leasing LLC, No. 09-1732 (6th Cir. Feb. 3, 2010)(citing Barnes v. GenCorp Inc., 896 F.2d 1457 (6th Cir.1990)). I think that framework remains viable following Gross, and it's now confirmed to be the framework in the Sixth Circuit.
There are plenty of arguments and counterarguments to go around though (just read through some of the briefs from the Wagner competition!).