White Sox reliever Matt Thornton is off to another great season: 1-0, 1.59 ERA, 10 SO, 1 BB, 5.2 IP. He's pitching even better than in 2008-09, when he was also dominant:
Multiple Seasons: 65+ IP, 2.75 ERA, 10.25+ SO/9, 2.75 BB/9 (Since 1900)
6 Randy Johnson 1995, 1999-2002, 2004
5 Pedro Martinez 1997, 1999-02
3 Billy Wagner 2002-03, 2006
3 Eric Gagne 2002-04
2 Tom Henke 1987, 1989
2 Trevor Hoffman 1997-98
2 Robb Nen 1998, 2000
2 Arthur Rhodes 2001-02
2 J.J. Putz 2006-07
2 Matt Thornton 2008-09
Thornton is unusual in that he is the only pitcher other than Arthur Rhodes with multiple seasons who was not a closer or starter. Thornton did take over as closer at the end of September last year when Bobby Jenks was hurt and converted 3 saves. Thornton obviously has the stuff to be a good closer, and he'll probably jump back into the role if Jenks is hurt or traded.
Thornton, who is 33, has a club option for 2011 with the White Sox. There's no reason to think that he can't be a fine closer, but it's worth noting that Arthur Rhodes was wasn't effective when he finally got his chance to close in 2004 with Oakland (at the age of 34). Fortunately, Rhodes did bounce back and become an effective set-up man again and is still pitching at age 40.
On the other hand, plenty of other pitchers without the track record of Thornton have made the transition effectively, such as Heath Bell, Ryan Franklin, David Aardsma and, so far at least, Jon Rauch. As great as being closer can be, though, seeing what happened with Mike Gonzalez and Jason Frasor in the past few weeks might be enough give one pause if they would even want the position.
Thornton's already in great company and will probably succeed if he gets another chance to close at some point. If he doesn't become a great closer, though, being the next Arthur Rhodes isn't so bad.