I've never been a huge fan of Eclipse, but since it's incredibly hard to argue with the price of LPCXpresso development boards with their removable SWD debuggers, basic support for flashing devices and debugging (size permitting) has just been added to the LPC1343 Code Base. Since the code base isn't based on CMSIS, and still runs from a Makefile, the process of creating a project and selecting debug or release mode is a bit different, but we've put together a fairly straight-forward tutorial on it here: Debugging the LPC1343 Code Base in LPCXpresso. Hopefully this will open the code base up to a few more people.
You will need to pull the latest version of the LPC1343 Code Base from Github to take advantage of LPCXpresso support, since the project files need to be more thoroughly tested before an official release. A final v1.0.0 release of the LPC1343 Code Base is right around the corner, though, wrapping up about 2 years of constant development effort on this library.
Note: Project files also exist for Rowley Associate's Crossworks for ARM, and the fully cross-platform and open-source CodeLite IDE, both of which are found in the /build folder. A very alpha DLL for CodeLite is also included in /tools, allowing you to program the flash and do step-through debugging via GDB Server with the Segger J-Link, but it's far from ready to see the light of day just yet.
Update 24-Mar-2012: LPCXpresso support has also been added to the LPC1114 Code Base as of v0.7.5.