It seems that DBLTX is mostly in very long maturities. That's why they were so successful recently, but it also makes them very risky if yields shoot up at some point, and they don't anticipate it.
Check this out:http://finance.yahoo.com/q/pr?s=DBLTX
Interesting. May be this information is out of date ? I looked at morningstar.com, and it clearly says that 84% (!) of their portforlio is 20-30 years:http://portfolios.morningstar.com/fund/summary?t=DBLTXAm I reading something wrong ?