One of the biggest comments on the recent SimCity announcement for the iPhone, other than being able to locate the SimCity International (link goes to the U.K. iTunes store) version of the game, was complaints that the game was crashing for some people.
This is not a new issue for graphically intense games for the iPhone and one of the solutions for solving this is very easy: Before playing your new game, reboot the iPhone.
Here's how to do it.
Update: As has been pointed out in the comments, a normal shutdown/startup process, or "soft reboot," should work fine: simply hold the power button down for 3 seconds and slide to power down. The "hard reboot" described here should only be used if your iPhone isn't responding to touches or the Home button -- it is not intended as regular maintenance. Thanks to all for the feedback.
[If your phone ever becomes completely unresponsive, you may need to do the 'hard reboot process described next -- but remember this is the equivalent of pulling out the power cord on your computer.]
Hold down the Home and the Sleep/Wake buttons until your screen goes black, as shown at right. After a few seconds, you should see a sliding bar for powering off the phone. Ignore it and keep pressing those buttons. After about 20 seconds or so, the screen will go completely black and the silver Apple logo appears.
Once your phone goes through the rebooting process, launch your game. You shouldn't have any further problems with it. One word of warning: Rebooting the iPhone does drain your battery somewhat, so if you're pretty low on power you may find yourself unable to use the phone until after you're recharged it.
I saw this tip posted on the App Store when I was purchasing Scrabble at the end of November and followed the advice and didn't have a single problem with Scrabble on my 1st generation iPhone. I repeated it with SimCity and, again, I'm not having any issues playing the game.
If that doesn't solve your issues, check out this list of iPhone troubleshooting tips.