So it came as a bit of a surprise (read: annoyance) when I tried to do just that--preview a layout file in Eclipse--the other day and it failed. Eclipse seemed to try to load the layout designer saying, "Eclipse is loading framework information and the Layout library from the SDK folder." Then it up and failed to load it. Over and over. You couldn't see the layout preview, nor the Properties pane or the Outline mode to add new controls to the layout tree.
You might be wondering why I bothered to investigate this since I could have just gone back to the XML and blown it off. Well, I needed a screenshot of the Layout preview mode for our new book. Nuff said.
At first, I thought it was just me overestimating the tool's abilities, trying to preview a layout that was too complex for the preview (this happens often when I include some of the more complex View controls in layouts). So I created a new project with its simple default layout file, and that didn't load either. So that wasn't it.
I did a quick search on the Android developer forums and a couple people were complaining on the forums about this over a year ago. One person suggested recreating your workspace. Well, that didn't work, either. I widened my search to the web and ran across a little note on Brian Klug's wiki, suggesting that the older versions of the Android plug-in JAR files might be hanging around, mucking things up. Thanks, Brian! This sent me in the right direction.
We've had a number of new revisions of Android lately. which have improved (greatly) the Android tool chain, that have had several new JAR file updates. Next, I had to find the android JAR files on my machine. First, I checked where Brian had suggested: c:\eclipse\plugins (basically, in the plugins directory of the eclipse installation path). As my luck would have it, they were not there. Instead, a quick file search for "com.android*.jar" showed that they were hiding in C:\Users\YourUserName\.eclipse\org.eclipse.platform_3.4.0_867647348\features.
With a little investigation, I found several older versions (0.8.x and 0.9.0) of the android JAR files installed with the new ones (0.9.1 is the latest for Android SDK 1.5 r3). I shut down Eclipse, deleted all the older JAR files, leaving only the 0.9.1 versions.
Specifically, I only kept these two android JAR files: