File Sharing Office Docs With Windows PCs

Question

I just upgraded my work computer, 24" iMac (pre-aluminum), to Leopard. At first I was thrilled to see the new file-sharing options in Leopard, as my computer has many company files shared. Before I was using a program called "Sharepoints" to share the files. That program does not work on Leopard.
Unfortunately, shortly after setting up the shared directory, I began getting complaints that people could not access the files in the shared folder. Here's what was happening:
They can get into the directory, and even open up the files (in this case it is excel files).
They can edit them, but when they save them they get the following error:
"The document was saved successfully, but Excel cannot re-open it because of a sharing violation. Please close the document and try to open it again."
But they cannot open it again.
When they try to open it again they get the following error: "Excel cannot access 'file.xls'. The document may be read-only or encrypted."
I can open and edit the document just fine on my iMac.
What I have noticed is that it appears to change some permissions of that file when they save it, but only on my username it now says "Custom" instead of "Read/Write". If I go to the parent folder, change the permissions back to what they were before, and apply changes to all enclosed items, they can open the file again and save it once (just like above).
What's the problem? We really need these files shared.

Answer 1

I just submitted an official bug report to Apple which I will reproduce below. I hope this helps someone somewhere.
Summary:
After upgrading (erase and install) my 24" non-aluminum iMac to Leopard, I am unable to share files with other people on the business network. The main culprit appears to be MS Office files.
Steps to Reproduce:
1) Choose the folder I want to share in System Preferences > Sharing > Folders.
2) Apply the appropriate permissions to the folder: (Me) -> Read & Write; owner -> Read & Write; staff -> Read & Write; everyone -> Read & Write;
3) Turn on SMB sharing and check the appropriate accounts.
4) Attempt to access an Excel file with Office from a networked Windows PC.
Expected Results:
Sharing should be normal. Other computers should be able to access, edit, and save all files as if the shared directory were on their own computer.
Actual Results:
5) File opens fine, edits normally, but upon save it issues this error:
(Office 2002): "The document was saved successfully, but Excel cannot re-open it because of a sharing violation. Please close the document and try to open it again."
(Office 2007): "Access denied. Contact your system administrator."
6) Document will not reopen again and if you try it gives following error:
(Office 2002): "Excel cannot access 'file.xls'. The document may be read-only or encrypted."
(Office 2007): "Access denied. Contact your system administrator."
Regression:
When examining the permissions on my computer after someone else has accessed and saved it (with Office 2002), my permissions are changed to this: (Me) -> "Custom". If I change the permissions back to normal, recursively, the entire process can be repeated.
If I create a rtf file and save it to the shared folder, it appears that it does not suffer from this problem. This leads me to think that perhaps it is an Office files only issue. Somehow Office is making changes to the permissions when it saves the file (in Office 2002), or does not understand the permissions (Office 2007). The problem exists with Excel, Word, and Powerpoint files.
Notes:
Related problem -- When I create an excel file from another Windows PC and put it in the shared directory on my Mac, I am unable to edit the file from my Mac. The permissions for the file are as follows: nobody -> Read & Write; staff -> Read only; everyone -> Read only.