As part of the Summer '13 Release, we have added the ability to use Dynamic Content in subject lines. Since Dynamic Content may contain HTML, a content check of the subject line was implemented to warn of invalid HTML and to ensure that deliverability would not be affected. This has had the unintended consequence of causing older subject lines that contain field merge <span> tags to generate validation errors if edited. Note that this does not impact the sending of existing emails. In general, if an email is opened in the editor and the subject line contains any angle-bracketed code such as a <span> tag, you will received a validation error and need to remove it.
To address this issue for field merges, remove the raw HTML <span> code and drag the field merge into the subject line from the field merge browser. The field merge will display properly highlighted, and will safely pass the content check. If you have a need to include any other content in your subject line between two angle-brackets <...>, the only workaround at this point is to create your subject line as Dynamic Content, then drag that Dynamic Content into the subject line as with the field merge case.
We are looking at options to relax the content checking in order to avoid any inconvenience.