Can everyone who has this problem please report it here
WP7.5 chooses Data connection over Wifi - Microsoft Answers
I really want MS to realise this is a major issue with a lot of people.
Seems like our effort on pushing thesevere bug on MS answers might have helped. At least it seems that Microsoft has acknodlegde the issue. Since we know that fixing of bugs is sometimes slow I'd recommend you keep adding your vote to "me too" in case you also encounter that problem.
I really hope it becomes a top priority for MS to fix this since in comparrision the much talked-about "disappearing keyboard" in my opinion was far less severe than this issue which one could cost real money and will surely make some users leave Windows Phone. I stumble again and again about users asking about "excessive data" and quite often it turns out that it was infact this issue.