Take a glance at the comments on a number of Rays' sites and one of the common statements is that Dioner Navarro isn't a good defensive catcher. That reasoning usually precedes advocating Shawn Riggans, Mike DiFelice, or even Josh Paul as the team's backstop backup. I'll cite Nichols' Law of Catcher's Defense here in regards to those three: "[A] catcher's defensive reputation is inversely proportional to their offensive abilities."
As you've seen on BTB my new posting niche involves graphs and spreadsheets; adding a much needed visual aid to what sometimes becomes a wall of text. For this perusal I'll do the same, and while defensive stats in general can't be weighed too heavily, and particularly not catcher's defensive stats, I headed over to The Hardball Times' catchers' defense pages just to see what the numbers said about Navarro.
Let me preface the rest of this by saying I feel Navarro's greatest detractor is his weight, I'd rather not make a fat joke or a food metaphor, but there's a reason one of his nicknames is the Gravy Train. Back on topic though, his delightful plumpness doesn't allow for the mobility behind the plate to get in front of some errant pitches or throws homeward bound, I wouldn't dismiss that losing weight would help extend his career ducking behind the plate either - taking a load off of his knees and legs can't hurt, after all look at the oldest catcher in the league, Brad Ausmus, he stands at 5'11", 190 - Navarro is 5'9", 205 pounds per the official site, both the small stature and Nichols' Law definition have extended Ausmus' career.
Moving on to the statistics as promised, the first thing I did was sort the catchers by innings caught and set an arbitrary floor of 700 innings to be considered for the list, naturally that netted me 28 catchers rather than the desired 30, but since the next closest catcher had 599 innings I decide 28 would have to do - 599 innings suggests that player caught roughly 70 games (IPC/9), 700 is around 77 games, the leader, Russell Martin caught 1,254 innings, or 140 games. Here are those 28 catchers: Martin, McCann, Posada, Johjima, Bengie Molina, Paulino, Varitek, Pierzynski, Rodriguez, Schneider, Martinez, Olivo, Laird, Lo Duca, Estrada, Navarro, Torrealba, Bard, Buck, Ruiz, Ausmus, Snyder, Yadier Molina, Hernandez, Zaun, Ross, Mauer, and the vastly overpaid Jason Kendall.
The average defensive catcher, based on the top 28 in playing time, caught 978 innings or about 109 games, had 0.74 stolen bases attempted against per game or 120 per season, threw out 24% of base stealers, averaged 0.35 wild pitches and passed balls per game or 57 per season, 746 put outs, 51 assists, and about 6 combined (fielding + throwing) errors.
Now obviously we're not going to look too far into innings caught, at least not for Navarro, he had numerous injuries last year, and it's hard to parlay certain things - a la a certain manager's obsession with playing a certain back up - into every situation. Instead we're going to compare the core defensive stats we've gathered: stolen bases attempted per game / season, caught stealing rate, and wild pitches + passed balls.
One problem I'm sure you notice is that stealing rates isn't at all a statement on the catcher's arm or ability, but rather a variable based on the pitcher and defense as well. If Greg Maddux is on the mound people can run all day, but if Andy Pettitte and his pick-off move are out there it's a different story. In fact, that's really the reason why judging catcher's defense is so hard - nearly everything they do, whether it be throwing out a baserunner, calling for pitches, or even that "art" of framing pitches is based heavily on someone else, either the umpire, batter, pitcher, or manager. Unlike just about every other situation in baseball catching is hardly a constant orientated position.
Despite that knowledge, here are the graphs of the top 28 and league average "lines" compared to Navarro in handy visual form for caught stealing rates:

Navarro is apparently slightly above average in gunning guys down; one out of every four attempted runners is thrown out. There are issues here of course, if Navarro only faces 32 steal attempts and throws out 8 is that more valuable than 16 outs of 64? After all that's giving up 48 more stolen bases rather than 24, yet getting the same percentage of outs.
I'm sure everyone knows that Russell Martin won the 2007 National League Gold Glove award for catchers - and I'm sure everyone also knows that Gold Gloves are pretty irrelevant, right Derek Jeter? Nichols' Law doesn't apply to Martin however, and since the two are going to be tied together because one or the other had to go, and Martin is apparently pretty good, here's a comparison of the two:

The numbers are added and the bar is broken down by percent; if the two combined for 10 errors and Navarro contributed 6, his bar would reach "60%", and so on. FE stands for fielding errors, TE for throwing errors, A for assists, and so on.
Is Navarro an above average catcher? The stats seem to suggest he's around there, but if there was ever a situation where statistics could lie, this would be it.