There is a nice improvement in Fedora 18 related to yum and PackageKit. It fixes a long-standing issue that while PackageKit is active (e.g. checking for system updates), any yum usage is blocked. You would see messages like this:
$ yum install foo
...
Another app is currently holding the yum lock; waiting for it to exit...
The other application is: PackageKit
Memory : 41 M RSS (447 MB VSZ)
Started: Fri Sep 7 09:27:08 2012 - 23:13 ago
State : Sleeping, pid: 1315
Another app is currently holding the yum lock; waiting for it to exit...
The other application is: PackageKit
Memory : 46 M RSS (452 MB VSZ)
Started: Fri Sep 7 09:27:08 2012 - 23:15 ago
State : Running, pid: 1315
...
It could last from seconds to dozens of minutes. Killing PackageKit was often the only choice. For me, as a yum-only user, this was really painful and annoying.
But it is no longer the case! As you can see in Bug 812938, Elad Alfassa implemented a patch to PackageKit-yum-plugin and Richard Hughes incorporated it upstream. If you run yum in Fedora 18 now, it will send a signal to PackageKit to cancel any background operations, run your yum command almost immediately and then resume PackageKit operation. The fix turned out to be quite simple, which is ironic, because this issue caused a lot of grievance among our users.
Elad, Richard, thanks a lot!
And the lessons learned for the rest of us is that if you have at least decent technical knowledge and something troubles you, it’s worth to have a look at it. The solution might turn out quite simple. (Personally, I really miss some team in Fedora that would focus on these small issues that cause a lot of discomfort).
Thanks again, guys.
Kparal's Fedora Blog 
“Personally, I really miss some team in Fedora that would focus on these small issues that cause a lot of discomfort”
Spot on.
Several years ago Ubuntu started its paper-cuts initiative precisely to address these types of issues. I have a bunch of similar “small issues” I would be happy to contribute to any initiative that gets off the ground.
Sometimes, Imagine easy solutions to some small issues. I downloads the package sources. Then I realise that the work isn’t as simple as I thought.
This is one of those subjective fixes. Ever since I have installed F18, I have faced this issue quite frequently. Yes, it doesn’t last for really long – but it does happen. Also noted at https://bugzilla.redhat.com/show_bug.cgi?id=812938#c26
The purpose was to shorten the waiting interval, you can’t get rid of it. But instead of waiting minutes as in Fedora 17, I usually wait just seconds in Fedora 18.