I posted this work around in another thread but thought I'd share it here too since I was also watching this thread.
I've had the same problem with being unable to access my Nortel voicemail system using the ** entry. After reading all of your posts I came up with a solution that works for me every time. I thought I should share this with all of you. First, you need to install a free application on your iPhone 3G that both records and plays back recordings. I used the YouNote free version but I'm sure many will work because the principle is the same. Then go to this website
http://www.dialabc.com/sound/generate/ and generate a sound of "**". I used the default settings of the program. Then, save the file in your computer, play it on your computer, and record the sound on your iPhone by using your recording application (eg YouNote). Then, place the call to you voicemail number and while its dialing exit the phone call screen, open the Record/Play App (YouNote), and at the appropriate time, play the recording. Then, just exit the App, open the phone icon and enter your password...you are in. This has worked for me every time. This is a little bit of a hassle but it's easier than the other suggestions I've seen. This will hold all of us over until Apple fixes this problem.