I am successfully processing payments with 5.1 and Authorize.net, but there are a few quirks:
I have 1 customer, to which the payments are associated. When I log in as that customer, I cannot see the payments. I see a screen with the correct number of lines, but only check boxes, no actual payment data. If I log in as admin, I can see the payments.
EDIT: (A day later) Now when I log in as admin I cannot see Customers nor Payments. The tabs present a page with only check boxes, no text. I am using FF 3.6.16
I have firebug installed, so if the xml is of interest to someone I can post it.