Readers of this blog will recognize my unhealthy fascination with loop quantum gravity and string theory. In my browsings of arxiv.org, I discovered a section of the physics area on popular writings in physics (what a concept: why can't we do this in theory CS too?). There's a nice article there by Rüdiger Vaas called 'The Duel: Strings Meet Loops' that explains the conflicts between the two approaches to unifying quantum physics and general relativity.
A related article is a Socratic dialogue between a fictitious professor and student on the pros and cons of string theory and loops. Bear in mind that this was written by Carlo Rovelli, one of the fathers of loop quantum gravity, so it is may be somewhat biased :)
And for those of you who say 'Bah ! Humbug ! Why should I care about such nonsense', I say 'Don't you want to understand John Baez's talk at SODA ?'