They don't exist in the Proboard Android app so everything is visible no matter what the original poster does. The text isn't even changed (quoted text is italicized) so it looks like part of a regular post.
Not much we can do about it, aside from accepting the fact that spoilers will be shown, or not use the app. Could always use the standard browser on Android devices, but I don't know if that will break the spoilers the same way (it shouldn't, as it works with practically everything except flash if you're on Android 4.0 like me).
It doesn't really bother me, just something I noticed. For the record, using firefox, dolphin and the stock Android browser on a tablet (3.1) all have the spoiler tags intact. I believe that spoilers are also viewable when you use the long reply button in a browser.
That way nobody has to accidentally log in and stumble into a spoiler topic in a spoiler board and read several posts that are full of those nasty, nasty spoilers. As soon as they see it's backwards, they can close their eyes and scroll away!
I actually read that fairly easily, up to the word "spent". That one gave me trouble. But it was all in a normal, legible font for me anyway! This won't work!
Why don't we make hundreds of invisible forums, that you can only get to with URLs. Then, in each spoiler tag, we can post the url to the hidden post in the hidden topic in the hidden forum. Oh, but we have to type the URLs BACKWARDS! That way, even when people accidentally find the forum and topic and the post and the tag and open it even though they know there will be spoilers...