The OP wants CS5 illustrator from CS3 indesign… so may not need the specifiler…? still sounds the same a using edit with from the contextural menu? but I don't have mixed versions installed so I don't know what id would list?

Nate, Im at home now where my little mac has CS2 & CS5… This opened a selected PDF in AI CS5 from ID CS2 ( if it was already running )… If AI CS5 was not running bridgetalk did NOT launch it like I would normally expect… The message just queues until I launch it… Strange? I tested with both versions of AI running too and it was OK. I had the PDF selected with the direct selection tool…

#target indesign
 
indesignMain();
 
function indesignMain() {
  
          if ( app.documents.length == 0 ) { return; }
          var doc, pdfPath, sel, script;
          doc = app.activeDocument, sel = doc.selection;
          if ( sel.length == 1 ) {
                    pdfPath = decodeURI( sel[0].itemLink.filePath );
                    script = illustratorEditPDF;
                    script += '\rillustratorEditPDF("' + pdfPath + '");';
                    btMessaging( 'illustrator-15.0', script );
          }else{
                    alert( 'Have 1 item selected?' );
          };
  
};
 
function btMessaging( targetApp, script ) {
 
          var bt = new BridgeTalk();
          bt.target = targetApp;
          bt.body = script;
          //bt.onResult = function( inBT ) {};
          //bt.onError = function( inBT ) {};
          bt.send( 5 );
 
};
 
function illustratorEditPDF( pdf ) {
  
          var doc, uIL;
          uIL = app.userInteractionLevel; 
          app.userInteractionLevel = UserInteractionLevel.DONTDISPLAYALERTS;
          app.preferences.PDFFileOptions.pDFCropToBox = PDFBoxType.PDFMEDIABOX;
          app.preferences.PDFFileOptions.pageToOpen = 1;
          doc = app.open( File( pdf ), DocumentColorSpace.CMYK );
          app.userInteractionLevel = uIL;
  
}; 

Im at work where I don't have 2 installs… It was a little weird for me in that if AI CS5 was not already running the message did not launch like it should… Just sat waiting I thing… When I did launch AI it open the file right away…? Odd… I commented out the call backs as they crashed my id every time… How I check this stuff is use…

$.write( script );

Just before the bt.send( 5 ); If the console result cut and paste to a new ESTK window = working script script = functionName; just strings it?. I find it easier in most cases to just pass a whole script as one string…

Try this way… Again it worked here…

#target indesign
 
indesignMain();
 
function indesignMain() {
  
          if ( app.documents.length == 0 ) { return; }
          var doc, pdfPath, sel, script;
          doc = app.activeDocument, sel = doc.selection;
          if ( sel.length == 1 ) {
                    pdfPath = File( sel[0].itemLink.filePath );
                    script = illustratorEditPDF;
                    script += '\rillustratorEditPDF(' + pdfPath.toSource() + ');';
                    btMessaging( 'illustrator-15.0', script );
          }else{
                    alert( 'Have 1 item selected?' );
          };
  
};
 
function btMessaging( targetApp, script ) {
 
          var bt = new BridgeTalk();
          bt.target = targetApp;
          bt.body = script;
          bt.send( 5 );
 
};
 
function illustratorEditPDF( pdf ) {
  
          var doc, uIL;
          uIL = app.userInteractionLevel; 
          app.userInteractionLevel = UserInteractionLevel.DONTDISPLAYALERTS;
          app.preferences.PDFFileOptions.pDFCropToBox = PDFBoxType.PDFMEDIABOX;
          app.preferences.PDFFileOptions.pageToOpen = 1;
          doc = app.open( File( pdf ), DocumentColorSpace.CMYK );
          doc.bringToFront();
          app.userInteractionLevel = uIL;
  
};

The problem is more likely in my script… I don't have a PC to test anything with… ( not that I would know how to switch it on, sorry )