All you have to do is to select the lines to comment out and then select Source Comment. A single-line comment, // , will appear in front of all the lines. To uncomment them, just select Source Uncomment.
126.96.36.199 Eclipse 3.0
Eclipse 3.0 also enables you to surround code in a block comment ( /*...*/ ). Just select Source Add Block Comment. To remove the comment, select Source Remove Block Comment.