Couple of bones to throw into the discussion -
(for those who don't know me, I'm aka Infidel on ArsForums, and I run www.wizarddesigns.co.uk)
Poly tops: I've been selling blocks with poly tops for six months and the only time I ever hear of them cracking is when people use the wrong fittings - ie. taper thread fittings with ptfe tape in parallel thread tops.
It doesn't take much thought to see how this can occur.
Mixed metal setups: I use (and have done for the past six months) an anodised alu res, two anodised HDD coolers and an aluminium cored radiator.
This is combined with 3 poly/copper blocks.
I use a combination of polypropylene glycol coolant, distilled water and UV dye and only changed my water after four months when my UV dye turned up (I had previously just dropped a highlighter pen in the res).
I have no signs of corrosion anywhere in my loop (although I obviously cannot inspect the radiator internally).