WORDPRESS UPGRADE TO 2.8 "BRIEFLY UNAVAILABLE FOR SCHEDULED MAINTENANCE. CHECK BACK IN A MINUTE.": WORK AROUND
I hate upgrades. WordPress is pretty good, but....
Well, I did my full-system backup. I backed up the database and all WordPress files via FTP. Then I hit the upgrade button. The page fully loaded but without the usual "success" language. I immediately attempted to go to my admin dashboard and received the following message: "Briefly unavailable for scheduled maintenance. Check back in a minute." Not good
I immediately tried loading the website. I received the same message. Doubly not good since visitors are sent away and so is Google indexing, etc.
I immediately went to the WordPress root via FTP to compare it against my backup but didn't have to because I instantly spotted the file ".maintenance." I transferred a copy to my hard drive and then killed the one on the HTTP server. Then when I tried loading the website, it came up and so did the admin pages.
I haven't gone any deeper than that yet but wanted to get this out there for others who will no doubt run into the same problem.
So, as a quick workaround, delete .maintenance in //
Ah, I see this upgrade has moved things around on my post-new.php. I wish upgrades would respect existing settings.
P.S. If this helped you, just leave a quick comment saying "Thanks" so I know whether or not to bother doing these types of posts. If you're paranoid about leaving your name, for this one post only, just say John Doe or Jane Doe says thanks.
UPDATE: June 15, 2009:
These pages have good information.
I found then for you after receiving your plea.
I have done the plugin-move method before. The issue with deactivating all plugins is remembering which ones you had enabled versus disabled before deactivating them. If you have many plugins, human memory can be a problem. Next time you change plugins, do a "save as" for the whole page. Then if you do have to deactivate everything, when you go back to re-enable, you'll have a list of how things were before.
Another issue is with turning each plugin back on one at a time. It's the right approach, but is very time consuming and extremely difficult to see which plugin is making which incompatible error anywhere on your site (admin or public). Manually check every main admin page and every theme file type.
There are lists on WordPress that do say which plugins are up-to-date for which WordPress version. I've used those lists before. I don't have a link handy, but I'm sure the 2.8 list wouldn't be too difficult to locate.
I'm sorry you've run into this problem. If you can't get things up and running from the suggestions on the linked page, try leaving questions in the help and forums at WordPress. I've found answers there before and left a few tips that I've discovered through trial and error and being led by the Holy Spirit in fact.
Another thing that helped was getting over trying to have every bell and whistle that the universe of plugins offers.
Many plugins are incompatible with each other too.
I have core plugins and don't go "shopping" for many any more.
Oh, one last thing, if you hack your plugins (I do it all the time), keep notes of what you've added and deleted. I cut and paste the actual code before and after and put it on the plugin's description line in between straight brackets [ ]. I started out making it bold, red, and capitalized too so I'd stop before upgrading right over my hacks (which would be lost to me unless my memory were as God's). This method also allows one to keep disabled plugins in the plugin folder pending automatic update-notifications. Some (very few) plugin authors don't get around to updating for months. Most plugins that don't get updated quickly fall by the wayside forever. I move those to a save folder just so I'll be able to see which plugins not to bother trying again if they haven't been updated. That helped when I was trying numerous plugins. A list would help, but I found it easier just to move the plugin and then consult the folder/subdirectory.
Peace and blessings to you.
Let us know how things turn out. Leave another comment about it to help others.