Omnigraffle is a very useful app. I've been using the desktop version on and off for a little while now. In fact, I love all the Omni Group apps on desktop and iPad.
That being said, I am not aware of any apps capable of displaying files in the native .vsd format. Omnigraffle wont do you any good unless you have the desktop version (Mac-only).
My solution has been to print/save as PDF and store in Dropbox (http://bit.ly/9c8jtA
) or other iThing-accessible cloud storage.
Another future solution would be Visio Services in SharePoint 2010 (provided you're in a SharePoint environment).
FYI, iPhone/iPad-compatible file formats include:
Images (.jpg, .tiff, .gif)
Music (.mp3, .aiff, .m4a, .wav)
Movies (.mov, .mp4, .m4v)
Microsoft Word documents (.doc, .docx)
Microsoft PowerPoint presentations (.ppt, .pptx)
Microsoft Excel spreadsheets (.xls, .xlsx)
Adobe PDF (.pdf)
Keynote presentations (.key)
Pages documents (.pages)
Numbers spreadsheets (.numbers)
Web pages (.htm, .html)
Text and rich-text files (.txt, .rtf)