Google Adsense Team has acknowledged a potential bug in Google Accounts related to changing password. The problem is related to the recent AdWords migration to Google Accounts.

If Adsense Publishers try to change their Google Adsense password from the Google Accounts interface, the password is changed for all other Google services (like GMail, Orkut) except Adsense service. The publisher may not be able to even enter Adsense service without the help of a Google Adsense support member. Read detailed description of the Adsense bug.

The Google Adsense Support sent an email admitting the problem
Thank you very much for bringing this issue to our attention. Please note that because of our recent AdWords migration to Google Accounts, certain AdSense accounts may have migrated to the Google Account system. Therefore certain AdSense publishers must, in fact, change passwords and other account information from the Google Accounts page instead of the previous AdSense page.
Adsense publishers can use the "Forgot your password?" link for resetting their password, whether or not they have migrated.