What's the difference between the Sony Ericsson Xperia Active ST17i, the ST17i-BKW, ST17a, and the ST17a-BKO? I am interested in buying Sony Ericcson Xperia Active (unlocked) but there are several choices and I can't figure out which phone is which. Can someone explain to me the difference? Thanks!
AFAIK not much other than the frequency bands they operate on:

ST17i: (i = International version) Dual-band

UMTS HSPA 900 (Band VIII), 2100 (Band I)

GSM GPRS/EDGE 850, 900, 1800, 1900

ST17a: (a = American version) Tri-band

UMTS HSPA 800 (Band V), 1900 (Band II), 2100 (Band I)

GSM GPRS/EDGE 850, 900, 1800, 1900