http://bugs.winehq.org/show_bug.cgi?id=19653
--- Comment #4 from Jeff Zaroyko jeffz@jeffz.name 2010-02-23 16:21:45 --- (In reply to comment #3)
http://www.leenukes.co.uk/?p=109 claims to have a workaround:
A less convoluted workaround is just to browse to http://store.steampowered.com in any browser outside of Wine and complete the purchase after signing in. Once completed the purchase automatically appears in steam.