I recently upgraded to Intellij-12 and xmlns:ivy="antlib:org.apache.ivy.ant" is no longer recongized

You need to configure the external resource so IDEA can parse the schema associated with the namespace declaration.

1. Put your cursor on the namepace declaration (the part that is highlighted in red)
2. Type ALT+Enter or ⌥ ↵ and select "Manually Setup External Resource"
   
3. Select the location of the ivy.xsd file. You can map to any location. For example to the ivy install on your file system. Alternatively, you can create a directory somewhere, copy any schemas and DTDs to it, then map to the schema in that directory. It's all up to how you like to organize your system.
4. Click OK and you should be in business.

You can alternatively configure this in the settings dialog. You can also edit and previous settings.

• Settings > [Project Settings] > Schemas and DTD