I've got a WordPress 1.5 install running at Dreamhost right now. Everything is working smoothly, except that Trackback's are not being sent out when I post. I get no error message, just nothing ever shows up on the post I'm linking to.
I am having no problems sending out RPC pings to such as Pingomatic. I'm also having no problems receiving trackbacks.
I've searched the archives here, and everything I've found seem to refer to either Alpha or Beta version of 1.5 or earlier 1.2 versions. The fixes there haven't been tried because the code that's changed doesn't look identical in 1.5 and I don't know enough to debug it.