I've just used the Software Update feature built into iOS 5 to update my iPhone and iPad, and it works a treat.
It's simple, straightforward and the download is far smaller compared to downloading the entire firmware package through iTunes.
Note: This is an iOS 5 only feature.
Here's how it works:
Click on Settings > General > Software Update
The iDevice will now check with Apple to see if there's an update. Today I find one.
Note that the download is a svelte 44MB, as opposed to several hundred megabytes if you download the update through iTunes.
Click Download and Install. This kicks off the process.
Read the terms and conditions, or forever live in fear of what you've agreed to!
If your battery is below 50% and you're not connected to the power source, the update won't install. Otherwise, you can ignore this message.
From this point the handset reboots and the update is installed. When you're handed back control of the iDevice, the OS has been updated.
Whole process took about 5 minutes.