Don't get me wrong, this is the pinnacle of the year for me, but not exactly for the same reasons as Chris (but I do enjoy the races). This is the time of year that the barely-UCI legal all carbon climbing machines stay in the trucks in favor of the cobble machines.
Carbon fiber wheels were more common at Flanders this year than in years past, thanks in part to a redesigned Zipp 303 with a wider rim to accomodate the larger tubulars used by the top pros on the cobbles (24-27mm). The 303 was also notiably beefier than the standard model. There where still plenty of alloy box rims out there (and I'm sure they wil be in full force at PR), but the new Zipps would make a nice Christmas gift (hint..Chris). I liked seeing a rim designed with larger tubulars in mind (cyclocross anyone?).
The Cervelo RS was used by Roger Hammond in Flanders and may be the team bike this weekend. I have seen the RS up close, and it looks ideal for Roubaix with the compliant stays, beefy bb shell and longer head tube to allow for a more upright cobblesque position. The R3 won a couple Roubaix's already, so the RS has an excellent bloodline.
Shimano's Di2 electonic shifting will be out in full force as well, and some bikes were running an extra shift..er.. switch (for lack of a better term) below the bar tops to allow for shifting from the more comfy position. To that end, I am sure there will be more than a few bikes with cross brake levers on the bar tops.
In looking at some of the Ronde bikes, I was surprised to see a lot of metal front derailleurs. Campy bikes were using Chorus 11 front derailleurs (metal), and SRAM Red ubikes had a special all metal front derailleur as well. Personally I have been running an all alloy Record 10 on my race bike as of late (merely for asthetics), however I have noticed how crisp and stiff the shifting is. Maybe the pros already figured out that they don't need to save the 5-6 grams by using a carbon fron d cage.
I am anxiously awaiting P-R for the action, the cold Chimay and of course the tech talk.