Just wanted to check here before I go open a bug...
I just upgraded to 4.1, everything seemed fine, but I encountered a .docx document this morning, and got the dreaded 'General I/O' error.
I then tried a bunch of different XML documents (.docx, .xslx, and .pptx), and every one resulted in the same error.
These are all docs that opened fine in 4.0.4
Will go back to 4.0.4 and confirm it resolves the problem...