Um... Can you provide a url where it doesn't work? The search form on this page seems fine:
http://skins.b2evolution.net/a.php?skin=TicTac

I just tested it on my skintest site and sure enough it doesn't work. I searched for a word that was obviously present in a default post and it could failed to return any results. I then checked "use extra-path info" and found that this corrected the problem. Note that the skins site uses the extra-path thing, and this is not the only thing that does not work in a packaged skin without that feature enabled. In the Ality skin if you access the full archive list you will get an empty center column unless extra-path is on.

I did not test with the change identified by radio and using extra-path info, nor did I revisit Ality for possible implementation of a similar corrective action.

Ahha!

It's got an action value of something like "blah.com/index.php?blog=1", and a method of "get".
So the resulting URL is "blah.com/index.php?blog=1?s=search+words&sentence=ALL"
So, it would work if you were using stub files or "default blog through index.php," too, because then the BlogURL is going to be a file without any params.

The form_formstart function checks the access method, and then appends the blog as a hidden input in the form if needed.

Thanks Isaac. I just tested radio's fix with extra-path and (of course) had no problem. I also went back to Ality and found that using the newest version of the archives link works with extra-path on or off. Bummer that this thread happened a day or two after sending Graham updated old skin zips. :'(