Gavin Pearce

If you are wondering why WordPress won’t automatically embed using the URL and oembed in 2.9 and above:

1. Check you’ve got the box ticked under Settings|Media -> “Attempt to automatically embed all plain text URLs”
   [I remembered that one! ;-]
   
2. Make sure you have set a maximum embed height and width in the same location. My max height was left blank, which WordPress defaulted to a height of “0″, meaning it didn’t show any auto-embeds.
   [This one I didn't!]

Nice! Always the obvious.