I'm a little surprised no one mentioned WebOS; the phone runs Linux and the PDK is in C (although as pointed out in the reply below, the PDK is only available on Windows and Mac right now).
I would also look at Meego (what Maemo became after it merged with Moblin), which you can code in C, C++, or Python (or really anything, although the native graphical interface is Qt).
The only issue with either of those is that you might be writing an app that no one will use (as one platform was still-born, and the second isn't yet born). That's the price you pay to avoid learning new technology though.
Right now? Nokia N900, without a doubt. Maemo is a lightly hacked Debian, the official toolchain is gcc-arm (admittedly without autoconf, but that's a blessing if you're in control of your own source), and deployment is straight-up apt. Plus it's a decent Python dev platform if your tendencies veer in that direction.
If the "lightly hacked" part gets in the way, you can even debootstrap a real Debian release onto the device and develop in that.
The hardware is decent as well; it certainly feels more solid than the HTC slide phones I've played with recently, and despite dropping it a few times it's barely got the shine worn off after 8 months.
All that being said, it's entirely unclear what the future of the platform is; Meego could sink without a trace in 6 months, and I don't know if Nokia are planning a successor device. In my mind they'd be insane not to, but sanity hasn't been Nokia's hallmark of late.
WebOS uses HTML/CSS for it's interface and uses Javascript for it's apps. Palm also has a PDK which allows you to write apps in C. Problem is Palm hasn't released it's PDK for Linux. Luckily some hackers have been able to get it to work. http://goo.gl/MnyQf
There really isn't one. Symbian might be the closest that's still out there. Android would be somewhat quirky at best, and Apple wants you to use their toolchain.
Having said that, take a look at Appcelerator's Titanium (appcelerator.com) or AppMakr (appmakr.com). They both provide tools that enable you to create HTML/CSS/Javascript screens and also intermingle with other scripting languages to produce a compiled app for desktop, Apple, Android, and others.
You don't give enough information for a reasonable answer. What's the app for and how do you want to distribute it?
For instance, if your app is intended to go into widespread distribution and make you some money, then the answer is either Android (suck it up an learn Java) or IOS (you should be able to develop on a Hackintosh).
OpenMoko, which is Linux-based and uses C for all the core apps. I don't think anyone uses it anymore, though; everyone just installed Android on their handsets.
Qt. That way you get Symbian, Maemo, Meego, and Linux tablets, like the Shogo (embedded applications, like the QooQ on that have been pretty successful.) Qt for Android, Windows, and iPhone seem to be coming along, albeit slowly.
http://arstechnica.com/open-source/news/2009/06/android-goes...
You can also cross-compile iPhone apps on Linux, however, you won't be able to sell it at their app store.
http://www.saurik.com/id/4
Another alternative is the Nokia N900 which uses maemo.
http://maemo.nokia.com/ http://maemo.nokia.com/n900/