Sorry it totally slipped my mind to post on this topic, I had several bookmarked to post back on.
Updating to 10.6 will solve this issue for you, for anyone who is unable to upgrade from 10.5 there is a "legacy" application which you can download from here. I am not sure how long this link will hang around for, so if you need to use this version (which won't auto update) then download it soon ;) Only download this application if you are stuck on 10.5, since it won't receive any more updates or support.
You will need to uninstall the Spotify version that you have installed before running the legacy application installer.
Again, sorry for the delay in posting this, its been a busy week!
Oh Spotify, you disappoint me. I've just started getting the "Spotify quit unexpectedly" message. For a few days, I've resintalled it every morning which seems to work fine. But now nothing at all will make it work. My MacBook's only about 4 years old running 10.5.8. Everything else is just fine thanks, so I'm not going to go through a costy upgrade just for you. I'm moving on....
I have the same problem... the application suddenly quit responding today when I tried to open it. The only thing I can do is "force quit" I do not want to lose all of my playlists on spotify. So I don't want to reinstall it. I am running on a Mac os x 10.6.8. Please help!!!