I would never buy a Mac especially not with all the reliability problems they have and mis-features like locking the SATA port to SATA 1 speed, disabling 802.11n on the older ones and requiring people pay to get the feature, etc. On top of that I would never run MacOS X, as I am a Linux developer, so why pay more (the Apple tax) for less hardware. I personally own a ThinkPad X200 which is much better and cheaper than anything I have seen from Apple.
As far as open phones go, there is really not much choice on that front. There is Openmoko which doesn't even have Edge/3G support or the T-Mobile G1 Android phone. It also looks like openmoko is dying off and they have canceled their phones planned to have Edge/3G support. Android looks promising but the phone still needs a lot more work and/or there needs to be more than one of them available. More Android phones should be available later this summer so perhaps it will gain more marketshare. So I am not surprised at all that currently people at open-source conferences are using iPhones. I recently bought one for myself after sitting on the fence about whether to continue to wait until a nicer Android phone became available. Hopefully in 2 years once my at&t contract finally runs out there will be much better Android phones available. With respect to at&t they are planning on releasing an Android phone as well but with crippled resolution only 320x240.