Known good steppings that are capable of 3ghz:
CCB1E 0550VPMW
CCBWE 0550UPMW (Some are crappy actually)
CCB1E 0605XPMW
CCB1E 0606VPMW
CCB1E 0608MPMW
CCB1E 0609FPAW
CCB1E 0609FPMW
CCBBE 0610DPMW
CCBBE 0615DPMW
Honorable Mention:
CCBWE 0530TPMW (2.9ghz avg.)
CCBWE 0546XPMW (2.85ghz avg.)
CCBWE 0609FPAW (Unconfirmed precise averages, but seems to be 2.8-2.9)
AVOID if you want to hit 2.8ghz+:
CCBWE 0534SPMW
CCBWE 0543TPMW
CCBWE 0546MPMW
CCBWE 0551 (Any packing code) (2.7-2.75ghz 1.55v)
CCBWE 0602 (Any packing code)
CCBWE 0607 (Any packing code) (2.65ghz 1.55v)