/usr/bin/sed on Solaris 10 and earlier does not support -i. I don't know if OpenIndiana / Solaris 11's sed support -i, but OmniOS uses gnu sed by default, so OI might also.
As I recall, AIX's default sed also does not support -i, but I no longer have access to AIX systems to test against.
Honestly, sed -i is so useful that I'd file a feature request if a sed didn't support it.
Its easier to just not use sed and use perl -pie instead. Ironically its much more portable to non gnu systems.