IMHO - the best (current) device you can use for a hackable handheld is the Nokia N900 - I wish I had one myself.
If you're just looking for a hacker project and not concerned about using a smartphone from the latest generation, you may also want to consider a Palm Treo 650 or Treo 680. Support for the 680 just went into the mainline Linux kernel, and others have ported to the 650 as well. You can pick them up pretty cheap on ebay. I've been playing around with both phones using a gentoo embedded environment, focusing mostly on reverse engineering the hardware with an eye towards developing drivers for the hardware components not yet supported by the kernel. I'm able to boot the kernel on both devices. The usb client hardware on the phone is supported, so I'm also able to ssh into the phone from my desktop via usbnet. At least one person has documented success at making calls on the 650 (not sure about the 680 - I haven't gotten around to it for either phone). See http://www.hackndev.com as a starting point. One of the principal guys on that site developed a Linux boot loader (cocoboot) for the Palm OS, and all their reverse engineering efforts are documented on the wiki hosted on the site.
