.mdi file viewer?

Sounds like something you should be asking microsloth at http://www.officeformac.com/productforums/. Details on their proprietary format is at http://office.microsoft.com/en-us/help/HP062193601033.aspx which leads me to believe they should be providing the converter.

Short answer: no.

MDI files are Microsoft Office Document Imaging - a proprietary MS format
that I believe they were trying to get accepted in favour of PDF format -
it's a similar sort of philosophy, except that where PDF works on just about
every platform available, MDI only works in Office for Windows, v2003 and
higher (in fact, the MDI print driver appears to have been removed from
Office 2007). They are far more than " Microsoft's version of the .tiff
format".

Unfortunately, there are no ways to read these docs on a mac - even the
venerable GraphicConverter can't handle them.

In other words, like Silverlight and Flash, another example of Microsoft trying to re-invent the wheel and refusing to adopt global standards.

No. MDI was a Microsoft proprietary version of the more general TIFF (Tagged Image File Format) format that was implemented in Office 2003 and 2007 (and has since been dropped in Office 2010 beta). It was intended for storing scanned images. It used a few custom tag-types, including one for storing OCR'd ASCII text (redundant, since TIFF already supported it). It's biggest difference was that it provided for 3 new data compression types - all undocumented. It's not clear what the purpose of the compression types were because they don't appear to make the files smaller; it's possible that they existed as part of an experiment or to intentionally stifle making compatible implementations.

You might consider using TIFF or PDF in place of MDI. Both are portable, and PDF will make for much smaller files (assuming they are not scanned).

This might sound obvious. Sometimes I do get these and to my surprise I find there is no way to look at them on my mac. The way to view is to go to my PC and deal with the document there. If you don't have a PC, you have to ask the originator to send a file you can actually read. That sounds reasonable to me, since it is not considered a standard file. The one that I just looked at again on the PC was an excellent representation of the original, so maybe they had a good idea that was not needed or whatever else they decided. I personally think Apple should be dealing with the thousands of applications that are never going to be in Apple format. I am finally getting around to installing Windows on my Apple computer, but feel it is a pain-in-the-neck.