Thanks for the response. My main browser is Firefox, but I have tried writing/editing my posts in all the browsers I have: Opera, SeaMonkey, Safari and IE6 (I have all these so I can make sure my websites are compatible), but the preview doesn't work in any of them. I do have a popup blocker, but it's not blocking my previews.
When I click on "Preview this post" the new window opens with all my blog header and sidebar and everything, but I get a "Sorry, no posts matched your criteria." instead of the post. (I don't get a 404 - Not Found.)
The link of the preview page, if it matters, reads something like:
I wonder if it has to do with my permalink structure which is
post-title/post-id and whether the /?p=200&preview=true isn't matching things up correctly. I didn't have a problem with the previews before upgrading to 2.5, so I'm not sure about this.
Thanks for the link to the hack. I'll definitely think about it, though I'm hoping for a fix that won't require having to install another add-on because I already have so many plugins I have to keep updated. I may end up trying it out though if I can't get this fixed soon.
Thanks again to both of you for your responses.