There's a certain snail that does just that. Though it isn't really symbiotic; it actually steals stuff.
Could I see a Wikipedia page, BBC article, scientific name, or other thing that lets me check up on that? Above all, so I can figure out what you mean by "that"?
Both that an animal would evolve to a more active or otherwise interesting lifestyle whilst being photosynthetic, and that photosynthesis could provide a substantial portion of such an animal's energy needs.
We could only use it for a single stage in life, Ie, the reproductive, or the early life stages.
More reasonable, but I'm still against it for the short term. Mainly because only the permanent version has been suggested, I suppose.
...Probably the early stages of life would be best. Although they'd still need to eat
some stuff, and would need to hide from predators more...
Very active organism that gets all of its food from photosynthesis
[url]dn16124-solarpowered-sea-slug-harnesses-stolen-plant-genes[/ur]
Salamanders that get a photosynthetic boost in early life
[url]http://www.wired.com/wiredscience/2011/04/symbiotic-salamander/[/ur]
Mother nature has this tendency to beat us to the punch and ignore verisimilitude concerns
Oh, look, bad links to the kind of thing I was looking for! I'll post now to avoid getting ninja'd again, then look at those.
EDIT: The salamander's seems to only be in the embryotic stage of development, which doesn't help promote the cause of lifelong photosynthesis. The sea slug (with the poetic and alliterative name of the Eastern Emerald Elysia) don't technically symbiotise with algae, and more to the point need to eat algae. I haven't been able to find anything suggesting that they rely on the photosynthesis to any extent, either.