A few comments from a hair-splitting telco geek...
While Japanese 2G (PDC) mobiles have had text messaging from pretty much day one with (non-)standards like J-Phone SkyMail and Docomo ShortMail, "SMS" as defined in GSM 03.40/MAP Phase 2 is a GSM-only technology and was unknown and unsupported in Japan until recently. Not only did these Japanese homebrew text messaging services not support sending/receiving messages to/from overseas, but they couldn't send messages across to other Japanese operators either. This is why text messaging in Japan was nearly totally supplanted by e-mail, which is cross-compatible across everything, as early as 1999.
Fortunately, Japan's pretty much hooked onto 3G these days, and the WCDMA/UMTS phones used by Softbank and Docomo do technically support GSM SMS, complete with international sending and receiving... although as noted above, whether the average Japanese user knows how to use these features is a different nabe of sakana. In additional, au (KDDI) uses the incompatible CDMA2000/EVDO standard and hence still doesn't support SMS, locking their users into their proprietary "c-Mail" instead.