This one is easy. From the market, get "SMS Backup/Restore" or something like that. Have it do a backup of your messages, and then delete all messages. Now you can access the Messages app again.
If you want to restore your messages, you can restore them all from the backup, but if this causes Messages to break again, you may have to restore the messages in stages (individual conversations, one at a time) until you find the sms that is causing the crashing.