Yes, if you want income and are willing to commit to hold a bond to maturity, you can hold the bond, get the scheduled payments, and get your principal returned at the end. US Savings Bonds are non-marketable (you cannot trade them, but can redeem early) bonds designed for this purpose.
The value of a marketable bond will vary over its lifetime as interest rates change and the bond matures. If you buy a 30 year US Treasury bond at par value (100) on September 1, 2011, it yielded 3.51%. If rates fall, the value of your bond will increase over 100. If rates rise, the value will decrease below 100.
How much the value changes depends on the type of bond and the demand for it. But if your goal is to buy and hold, you don't need to worry about it.