Arthur Rhodes, at age 40, is not only having the best season of his career, he's having one of the the best seasons of any reliever in baseball in 2010. If the season ended today, he would be one of only 3 pitchers in history with:
20+ IP, 0.70 WHIP or lower, 0.70 ERA or lower
1907 Harry Covaleski (Phillies): 20 IP, 0.65 WHIP, 0.00 ERA
1990 Dennis Eckersley (A's): 73.1 IP, 0.61 WHIP, 0.61 ERA
2010 Arthur Rhodes (Reds): 22.2 IP, 0.66 WHIP, 0.40 ERA
In 2009, the Reds had the 4th best bullpen ERA in the ML, but they are 26th this season. If Rhodes weren't having the season he's having, it would be a lot worse and the Reds might not be at the top of the division. The only run he allowed this season was a solo home run on April 10th to Jeff Baker (Cubs).
While the Reds had a good May (18-11), none of their other relievers stepped up (Micah Owings had the next best ERA at 3.65 to Rhodes' 0.00). It's too early in June to make any determinations (most relievers don't have more than one appearance yet), but the rest of the bullpen needs to improve if the Reds are going to continue competing with the Cardinals in the NL Central.