Is there a difference in having tires filled with CO₂ vs air?
This is a debate that gets picked up on every bicycle forum known to man (okay, that might have been exaggerating a bit). Typically you start to ask the question when you notice that the morning after you have used your CO2 cartridge, your tyre is ridiculously low. 

By no means am I a chemist but, as it has been explained to me, CO2 and butyl are more soluble than N2/O2 and butyl. Most tubes are constructed from butyl (fancy pants cyclists will sometimes use Latex tubes, haha). Due to the increased solubility you can expect that a CO2 inflated tyre will need re-inflating sooner than one you pump up by hand. 

http://www.madsci.org/posts/archives/may98/895552329.Ch.r.html

While the explanation above is dealing w/ escape from balloons I am going to assume that we can take that information and apply it to our discussion. 

Now, if i am incorrect (which I often am) please let me know!

I just noticed I didn't tackle the temperature issue. I do know that temperatures exceeding 50 degrees celcius (above 125 farenheit) can result in the potential for ... explosive fun, in regards to CO2 canisters. 

http://www.stayfill.com 

This company has a proprietary gas blend that won't leak from your tubes apparently ... ... ...