InfoPath Dev

I would like to use the data from a field as a column in my SharePoint library and to make this work, the field must not contain an Ampersand (&) symbol. What is the best way to ensure that users don't include special characters in a field?

You could use some code to get rid of the & character if someone enterd it to you textbox.
For example in an OnAfterChange event write the text from your textbox into a string and use the string.Replace() method and replace all & through and empty string or something.

Hope it helps

An additional complication that I failed to include initially, is that the users have the option of retrieving the data from a secondary data connection that could contain an ampersand, and what I really need to be able to do is convert the & symbol to "and" while executing the Rule that I am using to fill in the field from the secondary data source.

I used this formula to get rid of one of the &s, maybe you could cook up something with eval() to do it recursively?

concat(substring-before(field4, "&"), "and", substring-after(field4, "&"))