This disk is your os x startup disk and appears to have been partitioned by another utility
Yah disk utility is bound and determined to not let you (or itself) potentially erase any more data, especially if it sees the signature of another disk partitioning utility. Welcome to the Wonderful World of Macintosh Computing... sigh.

Probably your best bet is to back up your Mac and Windows data (if not already) and boot the Mac from the COMMAND-R keyboard shortcut and wipe and reinstall from scratch. Your Ubuntu installer has gone and done some things to the partition table that Apple doesn't like and it is likely not going to be recoverable easily.

You could likely haunt the Ubuntu forums as I am sure this has been done and CAN be done. Heck there might even be someone around these parts with that experience.  But if you need the Mac back up and running ASAP the quickest way might just be to repartition and start from scratch.