For months we have been having a problem with spell check in my environment where non-admin users were unable to run spell check within MOSS and receive the error "Spelling did not complete properly. If this problem persists, notify your system administrator."
In my case, users have Read access across the portal except to certain sub-sites where they are given Contribute and other permissions. As it turned out, the library for the custom dictionary that I had created also only had Read permissions for general users. Changing their permissions to Contribute solved the problem.
If you want to setup a custom dictionary in SharePoint you can do so on a Site Collection basis (i.e. one custom dictionary per site collection). To do so:
o Create a new Document Library at the root of your site collection called "Spelling"
o Upload a text document to this library called "Custom Dictionary.txt"
o In the text document place one term per line.
These terms will no longer be detected as spelling mistakes when you do a spell check through the UI.
These instructions came directly from: http://mosschampions.com/blogs/moss/archive/2006/11/21/Configure-a-Custom-Dictionary-for-Spell-Check-in-SharePoint.aspx
-Just make sure to give all users contribute access to this library!