I second that "YES".
Once you get your iPhone 4, you can just snap your Mini SIM down to a Micro SIM by snapping out the Mico section.
It will also speed up your activation on the iPhone 4, you will only need the quick iTunes tether to activate just the iPhone 4. It just needs contact Apple central and have it's S/N validated.
You can also choose to use the new pre-installed Micro SIM but as a reminder you can only have one Active SIM per line (aka tied a single number).
Dave