Version:           3.0.1 (using KDE KDE 3.4.2)
Installed from:    Compiled From Sources
Compiler:          gcc-3.3.6 
OS:                Linux

This is the second time I am having problems with valgrind because I have an Athon thunderbird.
It does have mmx-ext (instructuin like movntq, mfence) that are refered by intel zealots as integer SSE instructions.

I got an assert failer in
VEX/priv/host-x86/hdefs.c::2310

   case Xin_MFence:
      /* see comment in hdefs.h re this insn */
      if (0) vex_printf("EMIT FENCE\n");
      switch (i->Xin.MFence.subarch) {
         case VexSubArchX86_sse0:
-->         vassert(0); /* awaiting test case */  <--
            /* lock addl $0,0(%esp) */
            *p++ = 0xF0; *p++ = 0x83; *p++ = 0x44; 
            *p++ = 0x24; *p++ = 0x00; *p++ = 0x00;
            goto done;

removing the assert() solved the problem for me.

Well, now you have test case, so please stop bugging me!!!