Worst case you get one now and get to use Lion for quite some time before Mountain Lion comes out. If some of the features you find in Mountain Lion sound good to you then you can always upgrade for minimal (or no) cost. If you find Lion to your liking then you are off and running early.
I had the same decision about a month ago. Wait for the new model or just get one. I just bought one. I sometimes burn Linux DVD's and the internal DVD was a bonus there. With USB so prevalent for Linux these days it's not really an issue, but sometimes there are issues with USB so I like the DVD option without having an external DVD burner. I also find all the features in the current line plenty for what I do...surfing the web, writing, spreadsheet work, music, image and image editing. Sure, the new model may be slimmer and have better options, but so too will the next release after that. Worrying too much about it becomes and endless worry so I made the purchase and haven't looked back. My MB Pro is awesome and I really enjoy the elegance and simplicity of OS X.