My wife drives a '06 Mini Cooper S Convertible. We are expecting, due in October, and I have been researching everything I could find on strollers that fit in a Mini's boot. Those most mentioned, the Peg Perego Aria and the Quinny Zapp, both have negatives that prompted me to keep looking. the Aria only holds Peg Perego's $280 infant car seat, and the Zapp doesn't recline. Anyway, we finally found what we were looking for. The City Mini, by Baby Jogger, fits when one rear wheel is removed. It pops off in 2 seconds and then the stroller is so small that you would still have room for a diaper bag in the boot. Not only does it fit great, but the stroller itself seems nicer than many others that we tried. It holds 5 different types of infant seat, including Graco and Mico-cosi. I have read in other threads that the Graco Snugride fits in the Mini, so this combination should work.
I hope this helps anybody else who is in the same dilemma we were in!

 

I thought the Chicco Keyfit 30 fit the MINI quite a bit better than the Snugride. We are going to pair the Keyfit with the Graco stroller frame which left a decent amount of space in the back. I tried to cram the Graco Metrolite back there and there was no way without putting a seat down.

"The Kid Who Stil Has No Name" should be able to move to an umbrella style stroller by the time he outgrows the Keyfit. I have heard good things about the light-weight McClaren.

There are several other threads on here about babies and MINIs. Just do a search for car seats, infant seats, or strollers.