I have an Axiom Rackrunner - a bit of plastic that extends slightly out and down from the pannier rack, to which it's zip-tied. It's probably useless for anyone really close behind me, but it stops some of the mortar shots from raining down and prevents me from looking like a badger. Nowt on the front though - regular bike cleaning and laundry is sufficing thus far. Sadly, proper (well, semi-proper - I have insufficient eyelets on my bike) guards are in the "another thing to buy" pile at the moment.
Out of curiosity I did the maths for theoretical range (= v^2/g) of road crud for 15 and 20 m.p.h. (probably as fast as I feel comfortable going on wet roads on my commute), and it tops out at 4.6 and 8.2 metres respectively (ignoring air resistance) - that's for a 'target' about level with the bottom of the chain. So if you knock a little speed off in the wet, you'll cut down the 'danger zone' considerably even without guards.