+1 to Tyler's comment.
My personal philosophy on bonds isn't shared by many on this board, but it might be a useful food for thought.
When it comes to stocks, you get great diversification by buying a simple index fund. However, this should be balanced by low correlation or negative correlation assets like bonds, foreign stocks, real estate, etc.
When it comes to a bond funds ability to be negatively correlated, it makes a huge difference what type of bond you're buying. Most simple bond funds hold simple bonds with 2-10 year maturities from a variety of government and high-credit corporations. These funds typically don't move a lot in either direction regardless of what stocks do. While this isn't a bad thing, you can improve your bond portfolio in a variety of ways, depending on what you're trying to accomplish.
If you want to reduce your correlation to the market, stick some money in long-term treasuries. These are highly sensitive to interest rate changes, have a strong negative correlation to stocks, are highly volatile, and generally yield more than your typical broad bond fund. Don't put too much money here, but it can be an improved diversifyer.
In another direction, you could invest in high-yield bonds. These are actually somewhat correlated to the stock market, but they are not very sensitive to interest rate changes, and higher yields help smooth the volatility a bit. Again, I wouldn't put a lot of money here, but it would be a good diversifyer to your normal bond portfolio.
Other options include foreign bonds (including emerging market bonds), preferred shares (I prefer these in taxable portfolios over high-yield bonds), and Muni bonds.
I personally believe most sizable portfolios (maybe $100K+) should have some element of these bond funds in there. Remember, unlike stock funds that hold a little bit of everything, your broad bond fund will likely exclude most of these bond types completely from their holdings.