I posted a separate thread on SIP client.
What I found most useful on my son's ipod touch is the Skype client. ipod does not have 3G so is connected to wifi. With a earphone and speaker plugged in, it works great as a telephone device when I am home with wifi coverage.
I think this would be really great to have on the Kindle.
I noticed that there is a Skype 2.2 beta for Linux:
for several different Linux distributions. The technical requirements are:
1 Ghz processor or faster.
256 MB RAM.
100 MB free disk space on your hard drive.
Microphone and speakers or headset.
Internet connection – broadband is best (GPRS is not supported for voice calls).
Video card driver with Xv support.
Qt 4.4.0 • D-Bus 1.0.0
PulseAudio 0.9.16 (optional)
BlueZ 4.0.0 (optional)
I don't know if it is even possible to port this.