I found a bug when testing Windows 7 64bit pro while using Dell printers. The error I found was that when you would print to the 5110 an error message would pop-up when you printed the document that said "An unknown error has occurred".
So, that's completely not helpful. I found that applying the following patch, which was for Vista 64bit fixed our problems. http://ftp.us.dell.com/printer/R149294.EXE
*I used the PCL driver in the above example for Windows 2008 x64 server