I want to backup my HTC Desire, but I don't have the USB cable handy for a while. Any Ideas?
You can backup to your SD card, then use a card reader, the take out the SD card and physically copy the data to your computer. Backing up to the cloud via Drobox is another option.
There are number of apps that will help you with this. This describes a way to manage this: http://www.addictivetips.com/android/clickfree-for-android-backup-all-data-to-sd-card-or-dropbox-cloud/
If your device is equipped with Android 4.0, and you've got the SDK installed on your computer, you might want to take a look at Full Backup of non-rooted devices (for how to do a full backup using ADB), plus adbWireless, WiFi ADB, ADB WiFi, and similar apps (which allow you to connect your device to your computer via ADB without the need of an USB cable). I didn't check all of them, at least some of these apps require root. But you didn't give enough information to figure out whether that's an issue.
If you don't care about a full backup, there are several other apps available on the playstore featuring, amongst others, some kind of backup, and connect via WiFi. Most of them only backup your
.apk files -- but some also offer several sets of data. On of those is AirDroid (offers e.g. to backup your apps), then there's Webkey (which requires root, but then e.g. offers to backup your contacts and call logs). There are a lot more options available, and I cannot list them all.
Then there's always the possibility to have some server app running on your Android device, and trigger actions directly on the device. If your computer runs Linux, you might be a bit familiar with SSH: Search the Playstore for sshd, and you find a bunch of server apps. Running one of those on your device, you can connect remotely via SSH, and directly execute commands from the command line (of course you need to find out which ones are suitable to you -- which very much depends on what you want to back up). Again, root on the device will prove very useful here. Setting up some other file server (e.g. FTP -- search the playstore for ftpd to get a list of possible apps), you can access the backup files for transfer to your computer.
You asked for Any ideas -- so there are some. No complete explanation, as that would get a bit long to do for all listed possibilities. But if you decided for one of them, and need more details: Simply ask a more specific question then ;)
P.S.: If that all sounds to complicated "for a while", simply go to the next store and buy some USB cable, they are usually not that expensive. This also is an idea to consider ;)