The option CONFIG_MOUSE_PS2_SYNAPTICS has to be enabled (at least) to make use of the driver. In recent kernels (i'm using 2.6.36-gentoo-r5) this option depends on CONFIG_EMBEDDED, so it is not always visible. The synaptics ebuild should check for it, too and notify the user if it is disabled. Now that I enabled it I can finally use my touchpad in a proper way.
Enabling CONFIG_EMBEDDED allows you to disable features that applications usually expect to be present in a kernel. Quoting kernel developers, "only expert users who understand the impact of the configuration changes they are making should enable it."
That noone should disable options in CONFIG_EMBEDDED is clear, yet I don't understand why the kernel devs made an not-so-much-expert option like synaptics depend on that and why it's hidden like that. Maybe a bugreport should be filed against kernel.org?
As long as you have CONFIG_EMBEDDED disabled it should all work.
Ok, this actually works without problems. Was just me not properly configuring synaptics and I also missed the synaptics output in dmesg.