Because XML is case sensitive, in InfoPath you have the problem that field may not be equal to another field during text comparison due to their case.
One way that people have been relying on is the TRANSLATE function, which requires you to write really crazy code like this:
translate( /my:fieldx, "ABCDEFGHIJKLMNOPQRSTUVWXYZ", "abcdefghijklmnopqrstuvwxyz" );
Except you usually need to do this for both sides of the equation.
A less well known function is actually the Microsoft extension msxsl:string-compare
Here you can use this to compare the current user's name.
msxsl:string-compare(my:a_user, xdUser:get-UserName(), "", "i") = 0
The string-compare function will return 0 based on whether the strings are identical. The 3rd optional argument is language, using an empty string will default to system. The 4th optional argument is case-insensitive.
The msxsl:string-compare is supported in both InfoPath 2007, 2010 and works in browser forms.
Check it out! :-)