The 220EX sounds like good advice. Surprisingly, I have only gotten redeye once with the popup out of a few hundred frames shot that evening. Back then I was shooting on a 350D and I hadn't yet purchased an external flash.. THAT was the incident that convinced me hotshoe flash was for me and that was more than 9 months ago. I do have a 430EX, just that I don't carry it all the time. But if I even expect to need it, certainly - I really do appreciate the results it delivers.
I do understand the reasons why the 5D did not come with a popup, and I do understand the need for ext flash, heck I am planning to pick up my ST-E2 when the budget allows so I can control two slaves off the camera, it will make my product shots much easier, being able to light from both sides of a light tent.
As a hedge against 'unexpected circumstances', which is really all I am asking for and why the discussion took place, the 220EX sounds great, small and light enough to have with you ALL the time. The best tripod, or in fact anything, is no good sitting at home, which happens as I leave the 430EX behind a fair bit when I don't think I'll need it.
Or maybe I should just carry my A530 or even worse *gasp* a camera phone.. now that (cam phone) is REAL blasphemy.