Microsoft Upsizing Wizard upsizes Access byte field to SQL Server bit column with 'Required' property set to False.
It is a bug in Microsoft Access Upsizing Wizard 97/2000/2002/2003.
Before upsizing to SQL Server 2000/2005.
After upsizing to SQL Server 2000/2005 and linking it back to Access
There is nothing you can do apart from fixing it manually after upsizing in SQL Server.
Set property 'Allow Nulls' to 'No'
See this article for more information on Access issues with bit fields