We were in the same boat 2 years ago and spoke to two consulting companies with packaging services, so take my recomendations with a grain of salt.
The company I liked was called Intrinsic Technologies ( http://www.intrinsic.net/ ). They had a deep set of talent and we shared a laugh over some of the packages that we were having difficulty with at the time (cough...Real Player...cough). Their sales and technical reps seemed to understand the ins and outs of building packages and I wish we had given them our business.
The other consulting company we talked to was a subsidiary of large computer reseller, we went with them over my objections. They had 2 guys in the entire consultancy who could build packages, one whose packages didn't work on our systems as built and another who felt building packages was beneath him. It was not a pleasant experience and the entire initiative got scrapped.