Do some (prestige?) accents swap /v/ or /f/ for /w/? I have only a layman's grasp of phonology and a poor ear to boot, but it seems to me that for many Chinese whom I am told have markedly standard pronunciation, /w/ and some type of labiodental fricative are interchangeable. For instance, listen to Echo's 认为 near the end of this episode of Popup Chinese (from about 7:20 onward): http://data.popupchinese.com/778/elementary-it-happened-one-night-traditional.mp3

Am I the only one who hears this? Could these two phonemes be in free variation?
I mentioned this phenomenon in an answer to another post here.  This sound, which is actually [ʋ] (labiodental approximant), was also discussed in this post in the Beijing Sounds blog.  Being characteristic of the Tianjin region (in my experience anyway), it's not standard, but many people don't seem to notice anyway.