Here's a look at what the three leading systems have to say about Dunn:
In ’04, Dunn was ranked in the lower 20% of all left fielders. In 2005, only 3 full time players were ranked worse than Dunn in left. In 2006, only Manny was ranked worse than Dunn. Partial season UZRs have been released for roughly the first quarter of '07 (representing a glimpse through a small sample size so they need to be treated cautiously). That being said, up to that point in the season, Dunn was ranked as the 4th worst left fielder in the majors with only Burrell being rated as worse in the NL.
Dewan’s plus/minus system:
2003: 18th out of 30
2004: 26th out of 30
2005: 28th out of 30
Three year rankings (2003-2005): Dunn 30th out of 31
PMR to runs:
2004: 36th out of 41
2005; 37th out of 42
2006: 34th out of 40
Dewan's converted Zone ratings (these aren't PBP based) covering the season through June are not favorable either. The same sample size caveat applies here as it does with all in season defensive metrics. That said, Dunn is ranked 128th out of 133 players who have played left field this season. Only two NL left fielders are ranked worse.
Defense ages, it doesn’t get dramatically better as a player matures through his late 20’s. A consensus of the gold standard defensive metrics suggests Dunn has been one of the worst defensive left fielders in the NL. It's reasonable to conclude his defense represents a drag on his value because even though defense in left field may be suspect in general, PBP-based metrics indicate that a super majority of the reds opponents have a defensive advantage in left.