An updated fc4 kernel was released recently based on 2.6.16. The old LVM2 functionality in lvm2 package versions 2.01.* is incompatible with this kernel: an updated package based on lvm2 version 2.02.* is required. This requires the libdevmapper in an updated device-mapper package version 1.02.*.
device-mapper-1.02.07-2.0 has been pushed for fc4, which should resolve this issue. If these problems are still present in this version, then please make note of it in this bug report.