I tried the free as in beer vmware-server on our new quad opteron and probably got exactly what I deserved when using tainted modules. :-)
general protection fault: e040 [1] SMP
<Jan/04 01:48 am>last sysfs file: /class/scsi_host/host0/stats
<Jan/04 01:48 am>CPU 0
<Jan/04 01:48 am>Modules linked in: ipmi_devintf ipmi_si ipmi_msghandler vmnet(U) vmmon(U) ipv6
ip_conntrack_netbios_ns ipt_REJECT xt_state ip_conntrack nfnetlink xt_tcpudp iptable_filter ip_tables
x_tables video sbs i2c_ec i2c_core button battery asus_acpi ac parport_pc lp parport st sg e100
serio_raw pcspkr ide_cd k8_edac mii cdrom edac_mc floppy tg3 shpchp dm_snapshot dm_zero dm_mirror
dm_mod sym53c8xx scsi_transport_spi 3w_9xxx sd_mod scsi_mod ext3 jbd ehci_hcd ohci_hcd uhci_hcd
<Jan/04 01:48 am>Pid: 2317, comm: vmware-vmx Tainted: P 2.6.18-1.2747.el5xen #1
<Jan/04 01:48 am>RIP: e030:[<ffffffff88395db1>] [<ffffffff88395db1>] :vmmon:Task_Switch_S1B1+0x183/0x976
<Jan/04 01:48 am>RSP: e02b:ffff8801e79c7bb8 EFLAGS: 00010282
<Jan/04 01:48 am>RAX: ffff820000000000 RBX: ffffc2000003d000 RCX: 000000000000e040
<Jan/04 01:48 am>RDX: ffff82000000e040 RSI: 0000000000000000 RDI: ffff8801e9bf6000
<Jan/04 01:48 am>RBP: 00002aaaada80a80 R08: 7fffffff00000001 R09: 0000000000000000
<Jan/04 01:48 am>R10: ffff8801e79c7e98 R11: 0000000000000048 R12: ffffffff8058e000
<Jan/04 01:48 am>R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000660
<Jan/04 01:48 am>FS: 00002aaaada80a80(0000) GS:ffffffff8058e000(0063) knlGS:0000000000000000
<Jan/04 01:48 am>CS: e033 DS: 002b ES: 002b
<Jan/04 01:48 am>Process vmware-vmx (pid: 2317, threadinfo ffff8801e79c6000, task ffff8801ee6fd040)
<Jan/04 01:48 am>Stack: 736282f99c4145dc 000000009d53f5e8 ffff8801e9bf6000 0000000000000246
<Jan/04 01:48 am> 000000008005003b 00002aaaabb65290 00000000b41c1cc3 0000006300005eaf
<Jan/04 01:48 am> 820000000000efff ef980ea576c5ffff
<Jan/04 01:48 am>Call Trace:
<Jan/04 01:48 am> [<ffffffff883994eb>] :vmmon:Vmx86_RunVM_S1B1+0x3f/0x1a8
<Jan/04 01:48 am> [<ffffffff8838c21e>] :vmmon:__LinuxDriver_Ioctl+0x387/0xd35
<Jan/04 01:48 am> [<ffffffff8027f6f0>] __wake_up_common+0x3e/0x68
<Jan/04 01:48 am> [<ffffffff8022e141>] __wake_up+0x38/0x4f
<Jan/04 01:48 am> [<ffffffff80260729>] _spin_lock_irqsave+0x9/0x14
<Jan/04 01:48 am> [<ffffffff802976dd>] futex_wake+0xc6/0xd5
<Jan/04 01:48 am> [<ffffffff803045f9>] avc_has_perm+0x43/0x55
<Jan/04 01:48 am> [<ffffffff8838daf7>] :vmmon:LinuxDriver_Ioctl+0x529/0x583
<Jan/04 01:48 am> [<ffffffff8030512d>] inode_has_perm+0x56/0x63
<Jan/04 01:48 am> [<ffffffff803045f9>] avc_has_perm+0x43/0x55
<Jan/04 01:48 am> [<ffffffff8026a78d>] monotonic_clock+0x35/0x7b
<Jan/04 01:48 am> [<ffffffff803051ce>] file_has_perm+0x94/0xa3
<Jan/04 01:48 am> [<ffffffff8838db74>] :vmmon:LinuxDriver_CompatIoctl+0x23/0x36
<Jan/04 01:48 am> [<ffffffff802d7230>] compat_sys_ioctl+0xc5/0x2b1
<Jan/04 01:48 am> [<ffffffff8025d54d>] ia32_sysret+0x0/0xa
<Jan/04 01:48 am> [<ffffffff8025d4e2>] ia32_syscall+0x1e/0x6b
<Jan/04 01:48 am>Code: 0f b6 42 05 83 e0 0f 83 f8 0b 75 0c 8a 42 05 83 e0 f0 83 c8
<Jan/04 01:48 am>RIP [<ffffffff88395db1>] :vmmon:Task_Switch_S1B1+0x183/0x976
<Jan/04 01:48 am> RSP <ffff8801e79c7bb8>
<Jan/04 01:48 am> <0>Kernel panic - not syncing: Fatal exception
<Jan/04 01:48 am> (XEN) Domain 0 crashed: rebooting machine in 5 seconds.
I fear I'll have to look into xen a bit more and use that in the meantime.
UPDATE: Turns out, it's currently impossible to do what I want:
I guess I'll just have to disable Xen for now and go with vmware until I have new hardware for the soon to be virtualized host. :(