I just created this patch which allows lame frontend to show more accurate bitrate during encoding, expecially when vbr. In fact, when encoding a bunch of my files it used to happen sometimes to get a different value for bitrate in lame and in mp3guessenc (slightly different, truth be told). The root of this discrepancy was the bitrate calculation in lame which is still good for cbr encodings, but for vbr ones it appears to be different from the usual rule of bitrate=(amount of data)/(duration).
In order to do this, I added a simple function to the lame library, `lame_get_bytes_so_far' which returns the encoded bytes up to that point in time. This is needed for a precise calculation of actual bitrate in real time.
When performing cbr encodings, the old bitrate calculation is still used.
Hope this is useful!