Can WGA writers write new media content (Internet webisodes and cell phone mobisodes) during the strike? There's disagreement, but my analysis is yes - if the employer (or purchaser) is not a signatory to the WGA agreement. WGA strike rule 1 only prohibits such writing for struck companies. And, new media writing is not within WGA jurisdiction, so writing such content for non-signatories is not prohibited.
Remember - this blog is not legal advice. Consult your lawyer or the WGA for a definitive answer.
Here's the relevant portion of the rule: "With regard to programming made for non-traditional media (such as the Internet and cellular telephones), this Rule prohibits writing services performed for a struck company in connection with new programming intended for initial release on non-traditional media and the option or sale of literary material for this purpose."