xf86-input-synaptics was enabled for i386 and amd64 architectures on 8 May 2011, which is -current, and will be in 5.0-release. You are running 4.8-release, which is one release back from the most recent: 4.9. See FAQ 5.1 regarding the release cycle and flavors of OpenBSD.
You have three choices:
- Custom backport this driver and its dependencies into your 4.8-release environment. The source code is available, and you are welcome to do this on your own. FAQ 5.3 will get you started.
- Install -current. You would either upgrade to 4.9-release and upgrade to the most recent -current snapshot, or, cleanly reinstall from the most recent snapshot.
- Wait for 5.0-release, scheduled to be published on or about 1 November 2011.