Guess it depends how you define efficient. If your goal is to gain marketshare from AMD with Celerons in $399 DELLs, knock yourself out.
I say why not? There is money to be made so why not take it?
HP recommending Opteron over Xeon?
Perhaps they are and I can't say I disagree with them when comparing to 32bit Xeons. They also intend to offer Xeon IA32E systems later this year in larger configurations. It was a nice win for Opteron but don't get too excited.