As many of you know, I am a big fan of open source software and UNIX and UNIX like platforms! So the perfect mobile phone should use an UNIX like OS and it should be made out of open source software!
Symbian is now open source but it is not UNIX like.
The iPhone uses an UNIX like operating system, but the user can not make advantage of it officially. And it's not open source at all!
The Android platfrom looks very promising. It's complete open source and it's a Linux operating system. However, no official phone for Android yet available. However you can already develop applications with the Android API, but Java only.
The OpenMoko project looks the most promising for myself. It's a complete open source phone platfrom as well and it's, like Android, a Linux. The advantage of OpenMoko over Android for me: You are not bound to the Java programming language, but you can use all kind of APIs including C, Python, probably Perl and C++ as well. And there is already the first running OpenMoko phone, called Freerunner, out to buy. However, the phone looks so damn ugly.

OpenMoko offers the most flexibility to a Linux user and programmer. An OpenMoko user should develop his own applications, or make use of the forming community. Probably, then the first phone running Android comes out, OpenMoko will get ported to it too. Maybe this will be the time I will get myself a new phone?
There are several other Linux based phone platforms I did not mention yet. That's because I don't know much about them yet. :)
At the moment I am using the Nokia N95. Well, it's not using an UNIX like OS and it still has a closed source version of Symbian on it. But it does it job well: Nice pictures, WLAN, SMS, phoneing, etc. It's a tool and not a toy for me. The perfect mobile phone would be also a toy for me (e.g. programming my own applications for it with fun in an environment I like).