Apple's Mobile Me cloud synching service is a pretty good way of keeping all your computers and your iPhone in step all the time, as long as you're not already using an Exchange service or Google Sync that is. Now Apple has made the Mobile Me service that bit more compelling, especially for the paranoid iPhone user.
Mobile Me now sports a feature called Find My iPhone, which, as the name suggests, allows you to find your iPhone, wherever it may be. So, assuming you've lost your phone, you can logon to your Mobile Me account, and bring up a map showing you exactly where it is. If the map is pointing to your house, it probably means you have to look harder, but if it's pointing to the bar that you passed out in last night, it's time to head back to the scene of the crime.
You can also send remote messages to your iPhone - so you could say something like "If you've found my iPhone please call xxxx xxxx and I'll come and get it", assuming that the person who's found it is the honest type who wants to return it. If not, Find My iPhone has another great feature that will ensure that none of your sensitive data falls into the wrong hands.
If you're convinced that you're not getting your iPhone back, you can remotely erase all the data from it. OK, so you've still lost your phone, but at least all those compromising text messages and photos have been nuked. And best of all, when you replace your iPhone, you'll be able to restore all your data from the backup on your computer from the last time you synched it.
Oh, there's one important thing to remember. If you're going to rely on Remote Erase as the last line of defence, you need to remember to have a lock code on your iPhone, otherwise the thief/finder can just disable the feature, leaving you open to a tabloid exposè.