I found their response time to be quite reasonable - never has it taken more than 24 hours for me, even with technical questions. You do not need to contact support to set up ListMail.
One thing I have found with DreamHost is they do not like very large lists (ie. 40,000 members) as they 'unfairly' monopolize server resources. If your list is under 10-20 thousand you should have no problems, but I'm just guessing and can't guarantee anything... Perhaps you should write them with some details about your web site operations and the size of your lists before you switch over.
Regards