<p>I'd like to develop a few applications for a device I own which has Windows Mobile 2003 on it, but I don't care to hunt down a copy of Visual Studio 2003 to do so. I'd like to know if the Mobile 6 SDK can be used for this purpose.</p>

<p>There's no upgrade path to newer versions of Windows Mobile for this device, and I doubt I'll be getting any newer devices any time soon. Just in case you'd suggest something like that...</p>

You can create programs for WM2003. As long as you can install the .net cf you will be able to run programs compiled under the compact framework. The list of requirements are:
From
microsoft.com
:
System Requirements
* Supported Operating Systems: Windows CE .NET; Windows Mobile 2003 software for Pocket PCs
* Supported Device Operating Systems: Windows Mobile Software for Pocket PC 2003, Windows Mobile 5.0 for PocketPC and Smartphone, Windows CE .NET 5.0 and higher.