Of course, with the new camera, an additional bag is needed. One that is small enough, yet big enough for the camera. Originally, I had the Lowepro Even Messenger 100 in mind. However, it was not on stock in my store. The Event Messenger 150 was clearly by far to big...
Finally, I decided to gamble and bought the StreamLine 100, which gave the impression of being somewhat on the smaller side, maybe even too small. As you see on Lowe's web-page, the bag is divided into 2 compartments by a Velcro attached separator. This separator has got 2 little pockets itself, one for memory cards and one which houses a microfiber cloth.
Very much to my surprise, the EOS M fits nicely in the bag, with either of the lenses, 22mm prime or 18-55mm zoom, fitted. The lose lens and the 90EX speedlite will find enough space to fit into the second compartment.
Stowing the camera with the 22mm prime is really simple, just through it in!
With the zoom lens fitted, thing get a bit more difficult. The best way for me is putting the camera into the bag with the display facing downwardly.
Very unfortunately, there is no elegant way storing a spare battery other than the zip-locked outside pockets.
Obvious downside of the the StreamLine 100: there is not protection against rain.