How To Make WordPress Check For Updates Immediately
WordPress automatically checks for plugin, theme and core updates once every 12 hours. This is normally fine, but it can become annoying when you know there’s an important update or bugfix available yet it won’t show up in your Dashboard until the next time WP gets around to checking for it. Of course, you can also install the update(s) manually, but surely that seems like too much work when you should be able to use the one-click upgrade function instead 
So I created Force Update Check, a small plugin that gives you the ability to make WP check for plugin, theme and core upgrades immediately. Just install it and go to Plugins -> Check Updates Now whenever you want to run the update check. Alternatively, you can also find it in the list of installed plugin in the Plugins tab and click the “Run” action link.
Here’s a screenshot of the plugin in action : 
You can click the “Found XYZ updates!” links to go to the appropriate Dashboard tab and view/install the available updates.
Download
force-update-check.zip (2 KB)
Requirements
- WordPress 2.8+
Just a quick note here, The Plugin/Theme/Core updates actually check every hour if you’re visiting their page, else every 12 hours.
(their page = the plugins page, the themes page, or the core update page)
You are correct in regards to the Plugin/Theme pages, seems I missed that when reading the source code in question. However, the core update page checks every time you load it, not every hour (I still included that check in the plugin for completeness).