Just bumping this again.
Even with UC 1.7, I am still getting these errors. 90% of the time, the store works perfectly. However, when these errors trigger, someone on the site is getting a WSOD while trying to check out.
We can't reproduce it here. But these errors are populating our logs.
[Tue Apr 28 12:44:43 2009] [error] [client XX.XXX.X.XX] PHP Fatal error: Call to undefined function uc_order_state_default() in /home/sites/xxxxxxxxx.com/www/pages/sites/all/modules/ubercart/payment/uc_credit/uc_credit.module on line 193