Just thought I would post my experience here for everybody else.
I deleted my copy of Hexic HD at the weekend. I was absent minded as I normally always double check to make sure I am not deleting Hexic HD when I do my regular quarterly cleanups of my hard drive. Anyway I contacted customer support on Monday 13th October 2008 and spoke with a representative.
The representative did not have a clue what I was talking about but I remained polite and carefully explained to him what I had done (i.e. deleted it myself) and what I expected him to do in order to resolve it. I explained to him that there is a page on Microsoft's Web Site about what to do if you have this problem.
In this case I said that the Web Site said that I should contact customer support and provide them my details including my xbox serial number etc.. I also said that according to Microsoft's Web Site the fix for this problem would be provided to me as a redemption code.
The representative at this point went and spoke with a supervisor. When he came back he said the supervisor knew what I was talking about and that I would have my query escalated to the supervisor and I would have a response within 24 hours.
Today, Tuesday 14th October 2008 I've just recieved my redemption code from Microsoft in order to re-download Hexic HD. After reading other peoples replies I think the trick here is always to remain polite, can be difficult, but it is important that you have your query escalated to a supervisor and correctly logged.
Plus it seems Microsoft seems to be aware of the issue from the email I recieved. The title of the email reads: "Xbox 360: How to replace a missing copy of Hexic if a customer deleted the game from the hard drive or had the hard drive replaced because of a console repair".
And remember the representative may not know what you are talking about. You don't really get access to the people in the know until you reach 2nd and 3rd line support. After all the representative is mainly there only to log your call and attempt to fix the problem with a prepared script.