A number of people have brought this issue up to me (I would thank you, but I simply am not organized with my emails, sorry). Basically, because the POI Utility custom tags were creating new CellStyle objects for each cell in the Excel sheet, Microsoft Excel was going over the number of formats that it could use. Apparently there is some upper limit, like a thousand individual formats of something, before it can no longer apply styles to new cells.
To fix this issues, I am now caching the cell styles based on a Hash of all the properties in the CSS plus the date/number formatting for that cell. That way, two cells that have the same CSS and number format (for example) will actually share the same CellStyle instance. This slows down the process since each style has to have its CSS properties hashed in alphabetical order, but at least it fixes the "Too Many Fonts" issue. I will try to work on optimizing it a bit, but this should do for now.
The new code can be downloaded from the POIUtility.cfc Project page.