First, I'll tell you I had a HUGE problem created by this issue. If that /var/log directory get's too many entries, you can't even list it!
I had a problem where (I think) the modem was installed incorrectly and there were tens of thousands of those messages created. I found that the newer E-Smith versions generally utomatically detect PCI modems and serial cards. They almost always seem to be detected as ttyS4 and ttyS5 (for dual-port serial cards).
I added a cron entry to periodically clean out all mgetty.log.ttySx.* log files.
G