SAM ANDERS in 4x15: "[we resurrected] on a ship that we placed in orbit around the planet [Earth]"... "organic memory transfer came from Kobol, along with the 13th tribe"
(Note he didn't say it was a Cylon-specific technology.)
1. (my preferred theory) Humans can be resurrected. There is a resurrection facility operating that resurrects anyone who dies on or near Earth. Kara died and was resurrected there, where someone (Zak?) wiped her memory and sent her on her way.
1a. As #1 except it's "humans who've had intimate contact with a Colonies-Cylon" (Zak)
1b. As #1 except it's "humans who've had intimate contact with a Kobol-Earth-Cylon" (Sam)
1b. As #1 except it's "humans who carry an ancient gene that enables resurrection"
2. Half Cylons can be resurrected. Kara is half-Cylon. Father - Cavil? Saul? Daniel??
3. Ship of Lights deus ex machina