Something

this works:

mencoder 179.wmv -o 179.flv -of lavf -ovc lavc -oac lavc -lavcopts vcodec=flv:vbitrate=540:autoaspect:mbd=2:mv0:trell:v4mv:cbp:last_pred=3:predia=2:dia=2:precmp=2:cmp=2:subcmp=2:preme=2:turbo:acodec=mp3:abitrate=56 -vf scale=320:240 -srate 22050 -af lavcresample=22050 -lavfopts i_certify_that_my_video_stream_does_not_use_b_frames -nosound

this doesnt work:

mencoder 179.wmv -o 179.flv -of lavf -ovc lavc -oac lavc -lavcopts vcodec=flv:vbitrate=540:autoaspect:mbd=2:mv0:trell:v4mv:cbp:last_pred=3:predia=2:dia=2:precmp=2:cmp=2:subcmp=2:preme=2:turbo:acodec=mp3:abitrate=56 -vf scale=320:240 -srate 22050 -af lavcresample=22050 -lavfopts i_certify_that_my_video_stream_does_not_use_b_frames

Video stream:  417.699 kbit/s  (52212 B/s)  size: 247386 bytes  4.738 secs  95 frames
$ mencoder 179.wmv -o 179.flv -of lavf -ovc lavc -oac lavc -lavcopts vcodec=flv:vbitrate=540:autoaspect:mbd=2:mv0:trell:v4mv:cbp:last_pred=3:predia=2:dia=2:precmp=2:cmp=2:subcmp=2:preme=2:turbo:acodec=mp3:abitrate=56 -vf scale=320:240 -srate 22050 -af lavcresample=22050 -lavfopts i_certify_that_my_video_stream_does_not_use_b_frames 
MEncoder dev-SVN-r22065-3.4.2 (C) 2000-2006 MPlayer Team
CPU: Intel(R) Xeon(TM) CPU 2.40GHz (Family: 15, Model: 2, Stepping: 9)
CPUflags: Type: 15 MMX: 1 MMX2: 1 3DNow: 0 3DNow2: 0 SSE: 1 SSE2: 1
Compiled for x86 CPU with extensions: MMX MMX2 SSE SSE2

success: format: 0  data: 0x0 - 0x5dca2b
ASF file format detected.
VIDEO:  [WMV3]  320x240  24bpp  29.970 fps    0.0 kbps ( 0.0 kbyte/s)
[V] filefmt:6  fourcc:0x33564D57  size:320x240  fps:29.97  ftime:=0.0334
==========================================================================
Opening audio decoder: [dmo] Win32/DMO decoders
GetOutput r=0x0   size:24576  align:1
StreamCount r=0x0  1  1
AUDIO: 48000 Hz, 2 ch, s16le, 384.0 kbit/25.00% (ratio: 48000->192000)
Selected audio codec: [wma9dmo] afm: dmo (Windows Media Audio 9 DMO)
==========================================================================
** MUXER_LAVF *****************************************************************
You have certified that your video stream does not contain B frames.
REMEMBER: MEncoder's libavformat muxing is presently broken and will generate
INCORRECT files in the presence of B frames. Moreover, due to bugs MPlayer
will play these INCORRECT files as if nothing were wrong!
*******************************************************************************
OK, exit
Opening video filter: [expand osd=1]
Expand: -1 x -1, -1 ; -1, osd: 1, aspect: 0.000000, round: 1
Opening video filter: [scale w=320 h=240]
==========================================================================
Opening video decoder: [dmo] DMO video codecs
DMO dll supports VO Optimizations 0 1
DMO dll might use previous sample when requested
GetOutput r=0x0   size:230400  align:1
StreamCount r=0x0  1  1
Decoder supports the following formats: YV12 YUY2 UYVY YVYU RGB8 RGB555 RGB565 RGB24 RGB32 
Decoder is capable of YUV output (flags 0x1b)
VDec: vo config request - 320 x 240 (preferred colorspace: Packed YUY2)
VDec: using Planar YV12 as output csp (no 0)
Movie-Aspect is undefined - no prescaling applied.
[swscaler @ 0x8668a10]SwScaler: using unscaled yuv420p -> yuv420p special converter
videocodec: libavcodec (320x240 fourcc=31564c46 [FLV1])
[VE_LAVC] High quality encoding selected (non-realtime)!
Selected video codec: [wmv9dmo] vfm: dmo (Windows Media Video 9 DMO)
==========================================================================
Bus error

playback 

$ mplayer 179.wmv -ao null -vo md5sum 
MPlayer 1.0rc1-3.4.2 (C) 2000-2006 MPlayer Team
CPU: Intel(R) Xeon(TM) CPU 2.40GHz (Family: 15, Model: 2, Stepping: 9)
CPUflags:  MMX: 1 MMX2: 1 3DNow: 0 3DNow2: 0 SSE: 1 SSE2: 1
Compiled with runtime CPU detection.

Playing 179.wmv.
ASF file format detected.
VIDEO:  [WMV3]  320x240  24bpp  29.970 fps    0.0 kbps ( 0.0 kbyte/s)
Clip info:
 name: rick
 author: 
 copyright: 
 comments: 
md5sum: Parsing suboptions.
md5sum: Suboptions parsed OK.
==========================================================================
Opening video decoder: [dmo] DMO video codecs
DMO dll supports VO Optimizations 0 1
DMO dll might use previous sample when requested
GetOutput r=0x0   size:230400  align:1
StreamCount r=0x0  1  1
Decoder supports the following formats: YV12 YUY2 UYVY YVYU RGB8 RGB555 RGB565 RGB24 RGB32 
Decoder is capable of YUV output (flags 0x1b)
VDec: vo config request - 320 x 240 (preferred colorspace: Packed YUY2)
VDec: using Planar YV12 as output csp (no 0)
Movie-Aspect is undefined - no prescaling applied.
VO: [md5sum] 320x240 => 320x240 Planar YV12 
Selected video codec: [wmv9dmo] vfm: dmo (Windows Media Video 9 DMO)
==========================================================================
==========================================================================
Opening audio decoder: [dmo] Win32/DMO decoders
GetOutput r=0x0   size:24576  align:1
StreamCount r=0x0  1  1
AUDIO: 48000 Hz, 2 ch, s16le, 384.0 kbit/25.00% (ratio: 48000->192000)
Selected audio codec: [wma9dmo] afm: dmo (Windows Media Audio 9 DMO)
==========================================================================
AO: [null] 48000Hz 2ch s16le (2 bytes per sample)
Starting playback...
A:   6.2 V:   5.4 A-V:  0.839 ct:  0.240  49/ 49  3%  2%  1.4% 0 0 $<50/>