Cheftze: There are two concepts in play here. One is an issue of Hikon Likras Elokecha Yisroel. You always have to put on something special for Davening to show Kavod to HKBH. The second is that when one is in front of the King, one must dress appropriately. Therefore, one must be dressed in a manner which would be considered acceptable in front of a human leader. I always wear clothes for Davening that are just as nice or nicer as our class was told to wear when we visited the White House (we were supposed to meet the president but he was busy). That seems to be a fair application. I also make sure to put on something special just for Davening.
