I too use MobileMe exclusively across my devices for both my business and personal with two MM accounts. I have seen just about every MM problem there has been since 2003. I do use Dropbox with my kids (and shared folders). It is a fantastic app too.
The OP never mentioned the "iDisk not in sync error"... He said he was getting "The file is in use Error"...
If you are working with these docs from within your iDisk, you can expect to have problems... This has been an issue with MS Office docs / iDisk for a long time... The same thing happens with Apple's text edit app and others, unfortunately...
I have found that it is because of the auto save functions built into the apps. What I have done as a work-around is to ALLWAY's move ANY document you want to work on to your desktop (or anywhere you want to work LOCALLY on your machine). Make any changes to the doc you want then SAVE, then REPLACE the original document on your iDisk with the most recent version. It takes some getting used to but it works much better and every time...
MS Word gives you the option to save the "auto-saved" (backup) document where ever you want. By default the document is saved in the same place as the original. In your case this would be your iDisk and this seems to be the problem I have found... You could try to set Word Prefs to auto-save your Docs to your machine locally OR just turn off Auto-Save function to see if that helps your issue...
Text Edit (as of Snow Leopard) has open and save options in Text Edit / Preferences. By default, Text Edit docs are saved every 30 seconds and are supposed to "auto-delete" when you save the text edit document. I have found that the auto-saved document DOES NOT auto delete if you are working from within iDisk... iDisk will save both... Also, Text Edit DOES NOT give an option of where to save the "auto-saved" docs...
I have found the same is true for Rapid Weaver and many other apps (with auto-save)...
When I started ALLWAY'S working locally then, saving / replacing in iDisk every thing "just works"... I sync 4-Mac's, an iPhone and an iPad without issues... Also, automatic sync works fine...
Hope this helps...