As Jason and Nitass suggested:
http://devcentral.f5.com/wiki/default.aspx/iRules/log
There is a significant behavioral difference when the optional . is specified. When iRule logs messages without the facility and/or level, they are rate-limited as a class and subsequently logged messages within the rate-limit period may be suppressed even though they are textually different. However, when the and/or are specified, the log messages are not rate-limited (though syslog-ng will still perform suppression of repeated duplicates).
Aaron