Another limitation of the default database connectivity in InfoPath is the field size limit and no large data types, such as rich text.
You can migrate to SQL and web services and remove this limitation.
I think with code you could accomplish this. The only trick would be triggering on character entry while they are entering data. If this is accomplished then when they reach the limit it could insert an additional field.
You may just want to have multiple fields by default. I see in 2007 you can specify the limit and have it move to the next field automatically when the limit is reached. If you are inserting that may interupt this smooth transition.