Don't know if this will help with mail.app crashing during sync, but...
It appears that trying to sync bookmarks was causing the crash. I went through each item on sync, and only when I tried to sync bookmarks did mail crash.
I've synced twice with bookmarks unchecked, and everything seems okay (I sync with calendars, contacts, keychains, mail accounts, mail rules etc.).
Hope this helps.
FALSE ALARM: just crashed again when I closed Safari.
It appears that trying to sync bookmarks was causing the crash. I went through each item on sync, and only when I tried to sync bookmarks did mail crash.
I've synced twice with bookmarks unchecked, and everything seems okay (I sync with calendars, contacts, keychains, mail accounts, mail rules etc.).
Hope this helps.
FALSE ALARM: just crashed again when I closed Safari.