For me it would depend on the type of rain predicted. If it's a 50% chance of a steady downpour lasting a good portion of the ride, then I'd bring the jacket. That kind of rain can make you miserable and cold really fast, no matter what the expected high for the day is (keeping in mind that the high for the day might hit at 10 a.m. and then go steadily downhill, or might be the high predicted after the rain is over). But if it's 50% chance of passing showers, I'd skip the jacket unless it's very easy to pack. If it's a jacket you can ball up and stuff in a jersey pocket easily, then it would be worth having.

I wouldn't bring the iPod, but that's just me--usually I know no one at these sorts of events but always end up talking to other participants as we go. If I'm riding alone and need music to keep me going (which happens often enough) I sing out loud. Funny how no one ever wants to do these events with me......I can't imagine why...........

If the ride is supported and you know there will be patrols out to help with mechanical problems, then I'd bring just the usual tire changing stuff. Chances are you won't need it at all, and chances are if you DO need it, you'll only need it once. The very small chance that you'll need more than one spare is so tiny that it's only worth considering if you are going to be completely alone with no support. But there will be other cyclists around, and cell phones, and likely mechanical support, so probably not worth doubling up.

Have fun!

Sarah