See this article, which says:
Install Infrastructure Update http://support.microsoft.com/kb/953749/
and pay close attention to one small line written in the notes part of
the knowledgebase article:
To prevent the declarative workflows from impersonating the system account, the workflows will now impersonate the user who created the
This basically means that you should create a special account not
SHAREPOINT\SYSTEM, that is a site collection owner on the site
collection where the workflow is hosted, open the workflow in
SharePoint Designer while logged on as this new account and simply
save the workflow without modifying anything.
It will effectively change the account that associated the workflow,
which will allow it to start.
you should also run
stsadm -o setproperty -pn declarativeworkflowautostartonemailenabled
to enable workflows to start using the impersonation scheme