The reason I build this way is, I don't like jumping straight into Death cap. I feel really squishy so I go for Rylai's Crystal Scepter. If you're playing a AP heavy team, I get an Abyssal Scepter in place of the Void Staff. If you don't have runes, I suggest the void staff being the first on your list to get. Magic penetration is great. If you're playing an AD heavy team, I suggest going either Banshee's Veil for the ability absorb or a Frozen heart for the Health/armor and the cooldown reduction is good also.
http://i55.tinypic.com/ng7xxt.jpg

Bombs are excellent harassing, I either use bombs for creep kills since last hits are hard on Karthus. I usually put bombs under people while they're going for last hits to deny them.