I was wondering that myself. It seems that at the moment there is no support for b2evolution. I just put up a question myself on the flock support forum (http://community.flock.com/node/555). Hopefully there will be support for b2eveo soon or a plugin from b2eveo that emulates the way one of the other systems work.

b2evolution uses xmlrpc just like everyone else, so if they want to support it they should be able to without any modification to b2evolution.

In flock there's no ability to choose a categorie, just the option to type in tags. Maybe the missing category-support is the problem?

yes and no.
In b2evo you can set your default blog and default category.
Flickr uses that too..
So it must be possible..

If can edit /conf/_advanced.php around line 98 and change the following:
Code:
# by default posts will have this category
$default_category = 1;

Replace 1 with the number of the category you want to post to.

You're right! Flickr works properly. Hmmm.... :?: :?: :?:

I got flock dev preview 0.5 to auto discover it(on the 2nd attempt, I don't know what thats all about) as XML-RPC API type MetaWeblog. its looking for the xmlsrv/xmlrpc.php in your b2evo directory..however the lack of catagory support makes it not the most useable blog client...unless you only have one category

Sent in a comment about it though since I run 2 b2Evo blogs, here's hoping.
Flock rules though, I highly reccomend it.

It found even all my blogs, but not the categories. But it's work in progress. I think they will fix the bugs in some time...