from iPhone, can you search the flutter by key ? Or topic?

There's a Browse by Topic link on the Fluther home page after you log-in. It gives you a Topic tag cloud.

I'm guessing this is also available to iPhone users.

Minor correction -- the Browse by Topic link is on the Fluther home page and available to anyone, whether they are logged in or not. 8-)

I recently learned if you use www.iPhone fluther.com vs www.fluther.com it will be different. Trying the iPhone version of the web site restricts certain features like the one you ask about and the great answer feature and some others. Using the regular version on your iPhone let's you do more but it is harder to read.....try it both ways.