Yes, you can use rules to check if the value changed in the field and then
send an email using a connection. You can also do it programmatically. Right
click the control and select Programming > Changed Event and write code that
would send an email. Similarly, you can also submit the form programmatically
if you want to otherwise use a data connection.

SSA
MVP MOSS

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You can also use the alerts functionality to subscribe to the document
library and get an email if it is modified but this will not be as granualar
as something in the form.

You may also want to look at the Database Accelerator. The new version
coming out next week has notifications built in as well.