aspect ratio info lost after padding

hi

I noticed that the movie aspect ratio info is lost after applying padding
(after image zooming). 

Good case:

ffmpeg -i "bob1.avi" -target pal-dvd   -s 352x256 -aspect 4:3 -vframes 30 
"bob1.mpg"

$ mpginfo bob1.mpg 
  Mpeg 2 Program Stream File [Video]
  Muxrate : 10.08 Mbps
  Estimated Duration: 06:14.10s
  Aspect ratio 4/3 (TV)
  Not interlaced, chroma format: 4:2:0
  Size [352 x 256]     25.00 fps    9.00 Mbps

aspect ratio is 4/3. 

Nok case:

ffmpeg -i "bob1.avi" -target pal-dvd   -s 352x256 -aspect 4:3 -padtop 16 
-padbottom 16 -aspect 4:3 -vframes 30 "bob1.mpg"

$ mpginfo bob1.mpg 
warning: couldn't find any valid system header. I'm continuing anyway
bob1.mpg
  Mpeg 2 Program Stream File [Video]
  Muxrate : 10.08 Mbps
  Estimated Duration: 06:14.10s
  Aspect ratio 1/1 (VGA)
  Not interlaced, chroma format: 4:2:0
  Size [352 x 288]     25.00 fps    9.00 Mbps

See, aspect ratio changed to 1/1. The only difference between previous cmd
and this one is that paddings are added. 

Please comfirm/help. 

thanks

tong

PS. I'm using the last ffmpeg from cvs.