Our office has 50/50 Android and iPhone users. We're having this problem when iPhones, and iPads reply to emails sent from Androids. The mail accounts are using Active Sync for our corporate Exchange 2007 email server.
The problem only occurs if email is generated from an Android smartphone, and the reply is on iPad or iPhone. It is intermittent but frequent. I have seen the problem from both sides. I had an iPhone, and now I have the very _fine_ Galaxy Note. Unfortunately, co-workers with iPhones (about half the company) are less likely to respond to my emails due to the crashes. We're an IT integrator so we're in the field most of the time, and work from our mobiles about 85% of the time. Surprisingly I haven't heard of customers complaining about this yet. Perhaps it is only certain configurations. We're seeing it from Galaxy S IIs (Sprint), and Galaxy Notes (AT&T) both running the stock Gingerbread. No one has rooted their work phones.
Trying to use Google to search for discussions isn't easy either since the search terms are so common, as are threads discussing crashing Android email clients.
I still have my iPad and I just upgraded it to iOS 5.1. I'll do some more experimenting and try to learn more about the problem.