Oh, Janet, didn’t you learn anything from Nipplegate? From countless nearly nude magazine covers to her shamelessly eye-popping new video for ”So Excited,” Jackson is literally the most overexposed pop star of her generation. And while there’s no denying that sex sells, it’ll take more than cleavage to reverse the downward spiral of her career. The singer’s latest disc, 20 Y.O., trailed its vapid predecessor, 2004’s Damita Jo, by 85,000 copies in its first week. Her numbers remained strong enough to make the album No. 2, but they marked her lowest debut sales week since 1997’s The Velvet Rope — and a 51 percent dive from 2001’s All for You. So, what’s a 40-year-old sex symbol struggling to stay relevant to do? Letting her T-A-L-E-N-T overshadow her T&A would be a nice start. Besides, Justin Timberlake already brought sexy back.