Hmm. I'm really liking the EOS-M with 90EX acting as master and 580EX II off-camera as slave. What would be a good bag, not too big, that will fit EOS-M, EF-M 22mm, 90EX, and 580EX II?
I highly recommend Think Tank bags.
I just bought the Think Tank Mirrorless Mover 20. Haven't got it yet (should arrive the same day as my EOS M), but I LOVE all my other Think Tank bags so I'm assuming more of the same goodness. Seems about the right size for what you're asking.
[bump] I got the Think Tank MM 20 and my EOS M kit (with the complimentary 90EX!!). The bag is perfect for my needs. It will comfortably hold the camera w/22mm lens and wrist strap attached, Canon lens adapter, 40mm pancake lens, speedlite 90EX, iPhone, several memory cards, extra battery, my wallet and car keys.
BTW, for those who like Black Rapid straps: I have a Black Rapid SnapR 20 bag that I use when I carry my G11. The EOS M w/22mm and pinch lens cap fits into it, but it's a very tight squeeze. The strap, on the other hand, detaches from the bag and has male and female clip ends, a slider unit like on the bigger BR straps and a small version of the fastenR that screws into the cameras tripod mount. It's like a mini RS-7! And it fits into the Think Tank MM 20 along with everything else mentioned above.
Regarding the large flash unit question, I don't own a 580EX II, but my 430EX II will easily fit in the MM 20 along with the camera if I move one of the dividers and tilt the flash's head to the forward position.