Where did you all find the response code? Was it emailed out? I have tracked my order on their page and didnt find a close to the MTGOX-USD codes. Hopefully I can get them to email me mine...
Mine was in an email sent to me with the subject "Your recent transfer - BitInstant order 58d56464-08cf-48f5-b7fb-17xxxxxxxx"
I did a direct cash deposit at CVS to Mt. Gox account if that matters.
I did a cash transfer from CVS to MtGox also. I never received any emails though so maybe if i can get in touch with support they can send my confirmation email with my APIresponse code =/