I've had this problem for 2 months on both my 3Gs and now iPhone 4, so I don't think it is just a 4 issue.  Although it has gotten worse since I bought my 4 about a month ago.

I called ATT support this morning and my rep was clueless as to the problem and had no record of this being an issue for anyone.  She went on to tell me that the Microcell would only work if it were connected to a computer in the house??? Since the Microcell computer light "wasn't showing green"...that was a problem. She was insistent that the Microcell computer light had to be on, even after I explained to her that my Microcell wasn't connected to a computer just my router.

According to the ATT Microcell support rep, "in order for the Microcell to work you must have a computer in your house."   How can they be this oblivious to a problem that is so well documented?  Do they not have some sort of centralized trouble ticketing system?