Yeah, they haven't handled the recent mail upgrade very well. We had a similar issue on one of our dedicated machines with them, but it turned out that none of the mail was lost, it was just delivered in the wrong place. Having a very strong Unix background, I was able to locate it and send it along to the proper place for all our users without having to go back and forth with DreamHost support.
In our case, the accounts with mail filters setup got their mail delivered to the default backup location of /var/mail/username. Everyone else had their mail bounced back as undeliverable. In theory no mail was actually "lost" (accepted by the server but not actually delivered -- senders were notified of mail that was not delivered). I was able to go into the mail logs and notify those users whose mail bounced of the senders and subjects of all bounced mails so they could contact important senders and request that the messages be resent.
Hopefully the situation is similar with your machine and they can get it sorted out.
One suggestion regarding mail and domains in general: If you want reliable backup mail servers (and effectively redundancy for the few times things go fubar at your main site), take a look at
EasyDNS. For $19.95/year per domain, they provide DNS services and backup mail exchanges. If your primary mail server goes down for any reason, EasyDNS will accept your mail and hold it for up to 5 days while trying to forward it on to your server. It's saved my butt on a few occasions. The only hassle is that it won't integrate with your settings from the DreamHost panel (obviously) so you have to copy things over manually. But for the extra peace of mind, especially with mail delivery, it may be worth a look.
It's kind of funny (well, not funny for you) that this topic came up now. With the exception of this one almost-major incident and a few minor glitches, DreamHost has been rock solid for us over the years. Unlike many hosts I've heard about, they don't have frequent (or even infrequent) outages, server crashes, etc. Everything hums along nicely once it's up and running for the most part. Which is why just last week I recommended them to another client of mine who just got a dedicated server. I was a little worried because their dedicated server prices are a little higher than some of the competition, plus there've been these few issues. But after reading reviews of the other guys and all the horror stories, I consider myself pretty lucky with them. At least I know what I'm getting into! Plus, a little loyalty never hurt - the 4 guys who started DreamHost went to my college, a year after me.
