Leopold: When the SRD says that the Druid can only assume one form, unlike the standard version of Polymorph Self, it means that he cannot change to another form (except to his natural form) without using the ability again. One who casts polymorph self can keep changing forms as long as the spell lasts.
Furthermore, both Polymorph Self and Wildshape were errata'd, with the splatbook versions superceding the PHB versions (It says this explicitly in said splatbooks). In MOTW (where the updated Wild Shape is), it says "the druid may only adopt one animal form per use of the ability.
Another update to both poly self and wild shape is that you get all the new forms Ex abilities. For druids in elemental form, they get Su and Sp abilities, too.