I hear there weren't really any "problems," so much as that Steyr wanted to improve the trigger pull a bit. My M40 is an old-timer that never got the trigger upgrade, but it works just fine.
I'm not sure when the trigger upgrade became standard on the M9/M40 series. I always hear that it's the sub-10,000 serial numbered pistols that are suspect, but I could be wrong on this.
Perhaps you already know this, but the FAQ here has an interesting tidbit about the mfg dates:
"· When was my STEYR manufactured?
To check the date look on the inside of your frame by where the metal plate w/ serial # is. There is a little clock/dial w/ hand poiting to month (1-12) and 99, 00, 01, 02 etc inside the ring. "
I'd cleaned my M40 dozens of times without ever really noticing the date stamp on the frame! Thanks for the info, guys!