Actually, GroupWise is putting a date in all messages - I've looked at what the GroupWise IMAP server passes out the messages. The messages that come from the Drew mailing lists have a Date: field, but the Date field is missing a timezone offset. That might be enough to do it. That's not a groupwise issue, that's the way that the (non-GroupWise) mailing list server is stamping them and we may be able to fix that.
GroupWise does NOT put a "Received" field in internal GroupWise email - messages sent from one individual user via GroupWise to another. That doesn't cause problems on a Motorola Droid. If the Nexus One is relying on "Received" to find a timestamp and ignoring "Date", there's nothing we can do about that. That's not standards compliant, AFAIK, so I would expect Google to fix it.
If timestamping the mailing list messages isn't the problem with the Nexus One, you'll probably have to wait for Google to fix it. There's really no way for us to change the GroupWise behavior to work around it, and it seems to work fine with every other IMAP client we've seen.