I am currently working in 2010, and some of the databases that I have exported from were the 2003.  They always truncate at 255 characters.

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Reply from Richard Rost:

I just tested it in 2013, and it worked fine. I don't have 2007 or 2010 installed anymore, so I can't test those, but I do have 2003 on one of my machines still...

Just tested it with 2003 and it worked fine. I made a Memo field in an Access 2003 database. Filled it with about 400 characters of text. Right-clicked on the table, EXPORT... to Excel 2003 format. Loaded up the spreadsheet. Checked the LEN() of the field. It's 439 characters. Works perfectly fine for me.

Perhaps there's a problem with your export routine?