I have reported a number of times that I have a successful install of that "bulk"/Amazon Sandisk card which I paid $47 for last August. I had not read or heard anything about card issues and had just ordered the cheapest sandisk card available from my Prime source when I first registered at this site and started learning.
Here is an early post (see post #7) where I describe
the restoring/ what i had to do to get my Focus to integrate, and finally show in Settings -> About, the extra memory.
http://forums.windowscentral.com/focus/180118.htm#post1562705
My main advice is to
be patient and persist.
Note that this post is not meant to counter any compatibility information. It may be that those cards lead to easier installs or better reliability. But, my 36.9 install has performed flawlessly, including a successful, first-try install of the Mango/"7.5" update. I have never experienced a Focus or WP7 issue, prior to or since Mango.
Note also, that I installed the card on a fresh, new Focus. The device had/s never gone through any "NoDo"s or any other software improvisations. I have only installed updates when prompted by Zune. But, no, I don't know how histories affect installation or other prospects for the Focus -although I have my suspicions and biases, one or two of which may be partly accurate.
Best of luck with getting your install to work.
Edit: I don't know that the card listed as compatible at Sandisk is not my card. A prior compatibility list had been pulled at Sandisk, Microsoft, and Samsung sites when I was searching last August. I don't recall that circle-4 on the card I installed. BTW, here is the direct link to the Sandisk compatiblity page:
http://pct1.sandisk.com/NewSearch.aspx.