Mac uses decomposed form (vocal character + accent character) on file or folder names with accents while Windows uses composed characters (vocal with accent character). This, and Finder always translating file and folder names, renders opening certain files impossible. There are quirks too, like having two files with apparently the same name (case and everything), but beneath the difference is decomposed vs. composed characters.

For example:
1 - I opened Notepad in Windows, wrote "Windows" and saved as "D:\teste - Cópia.txt".
2 - I opened TextEdit in Mac, wrote "Mac" and saved as "teste - Cópia.txt" on volume Dados (same partition as D: in Windows). First thing you should notice is that it doesn't overwrite the other file.
3 - Explorer shows 2 files with apparently the same name. Opening each file (it calls Notepad) shows that their contents are different.
4 - Finder shows 2 files with apparently the same name. The preview pane shows their contents are different. Double-clicking always opens the decomposed-accented one ("Mac"), even if I double-click on the other. Copying the "Windows" file to the desktop copies the "Mac" one instead too.


Resuming:
1 - Finder can preview composed-accented files if the rest of the path doesn't have accents.
2 - Finder cannot preview files with a path that contains composed-accented files.
3 - Finder translates composed-accented file and folder names to decomposed-accented. Trying to open such files with the associated applications generates an error in the app ("File not found"). Finder cannot even copy files.
4 - Quirk, if both a composed- and decomposed-accented filename exist in the same path (with apparently the same name), it will open the decomposed-accented file if you try to open the composed-accented. This is a result of the previous point.

The oddest thing is that both Windows Command Line and Mac Terminal show the composed-accented file and folder names correctly and the decomposed-accented with a special character after the vocal. This tells me Apple wasn't that much consistent and is probably some left-over from older OSes.

I suggest "translation" between the filesystem and the OS, which shows file and folder names as decomposed-accented but uses composed-accented names for actual filesystem access, perhaps as another mounting option. Right now, I have to resort to Terminal to copy files and folders with accents created in Windows.

PS: I tried to open each file in Mac TextEdit using the open dialog instead of double-clicking in Finder, but the result is the same: it shows both files in the dialog, but it translates its name before passing it to the app, thus making it always open the "Mac" file.

PPS: Wtf was on people's heads when they created Unicode?! Either composed characters or decomposed, not both!

This is a known limitation of MacFUSE. See http://code.google.com/p/macfuse/wiki/DesignDocFilenameEncodingSupportForMacFUSE for a detailed discussion of the problem.

Thanks, that's exactly it.