This warning, first reported for 2.6.25-rc5-mm1
(see http://lkml.org/lkml/2008/3/12/444)
has now made it into mainline:

May 11 19:32:44 xenon kernel: [   29.017148] powernow: This module only works with AMD K7 CPUs
May 11 19:32:44 xenon kernel: [   29.607563] sysdev: class cpu: driver (c0439804) has already been registered to a class, something is wrong, but will forge on!
May 11 19:32:44 xenon kernel: [   29.611835] ------------[ cut here ]------------
May 11 19:32:44 xenon kernel: [   29.616182] WARNING: at drivers/base/sys.c:183 sysdev_driver_register+0x59/0xf2()
May 11 19:32:44 xenon kernel: [   29.622037] Modules linked in: acpi_cpufreq(+) speedstep_lib microcode firmware_class loop osst st sr_mod cdrom pata_acpi bas_gigaset snd_hda_intel gigaset isdn snd_pcm rtc_cmos ata_generic aic7xxx snd_timer rtc_core usblp snd slhc ohci1394 pata_marvell thermal ieee1394 rtc_lib i2c_i801 parport_pc intel_agp processor soundcore crc_ccitt iTCO_wdt button scsi_transport_spi iTCO_vendor_support parport snd_page_alloc agpgart i2c_core sg ext3 jbd mbcache linear usbhid hid sd_mod ahci libata uhci_hcd ehci_hcd scsi_mod dock usbcore dm_snapshot dm_mod
May 11 19:32:44 xenon kernel: [   29.648305] Pid: 2860, comm: modprobe Not tainted 2.6.26-rc1-testing #1
May 11 19:32:44 xenon kernel: [   29.653156]  [<c0129d09>] warn_on_slowpath+0x41/0x6d
May 11 19:32:44 xenon kernel: [   29.658026]  [<c0120032>] ? push_rt_task+0xb1/0x1ab
May 11 19:32:44 xenon kernel: [   29.664013]  [<c012a75a>] ? vprintk+0x3a5/0x3c5
May 11 19:32:44 xenon kernel: [   29.670995]  [<c010b7a4>] ? native_sched_clock+0xe0/0xff
May 11 19:32:44 xenon kernel: [   29.675849]  [<c010b7a4>] ? native_sched_clock+0xe0/0xff
May 11 19:32:44 xenon kernel: [   29.680678]  [<c0143881>] ? put_lock_stats+0xd/0x21
May 11 19:32:44 xenon kernel: [   29.685465]  [<c012a78f>] ? printk+0x15/0x17
May 11 19:32:44 xenon kernel: [   29.690406]  [<c026a57a>] sysdev_driver_register+0x59/0xf2
May 11 19:32:44 xenon kernel: [   29.696845]  [<c02a8fe3>] cpufreq_register_driver+0x80/0xfc
May 11 19:32:44 xenon kernel: [   29.701892]  [<faafe034>] acpi_cpufreq_init+0x34/0x3a [acpi_cpufreq]
May 11 19:32:44 xenon kernel: [   29.708279]  [<c014d441>] sys_init_module+0x1883/0x1a34
May 11 19:32:44 xenon kernel: [   29.713176]  [<c0106b63>] sysenter_past_esp+0x78/0xd1
May 11 19:32:44 xenon kernel: [   29.718586]  =======================
May 11 19:32:44 xenon kernel: [   29.723414] ---[ end trace da2515dc0122d0b1 ]---

ts@xenon:~> uname -a
Linux xenon 2.6.26-rc1-testing #1 SMP PREEMPT Tue May 6 19:52:43 CEST 2008 i686 i686 i386 GNU/Linux

Intel Pentium D940 processor on Intel DQ965GF motherboard. config attached.

HTH
T.