Hi Carol,
Your question is very timely. After 10 years of InfoPath experience (building the product at Microsoft followed by being an MVP here at Qdabra), we have finally figured out how to do this. We have a scanner solution that requires scanning barcodes into a register checkout form. Obviously, for ease of entry, we want to automate this.
InfoPath doesn't allow you to treat a carraige return like a tab. However, in 2007, there is a feature that allows you to move to the next control when the field limit has been reached:

It only works if hte field is fixed widht and that may be an issue for you, but you could split the field up into smaller unit fields and add validation to make sure the thentries are valid.
Hope this helps in some way