It sounds like the Ubuntu beta is now using GRUB2 as the default boot loader. It's configured differently from the older GRUB Legacy, with which you may be more familiar. I've been experimenting with GRUB2 myself lately, and I've found that it's possible to use it to directly boot OS X -- see
this thread for details. It's conceivable that your configuration is using this approach, which bypasses the usual OSx86 boot loader and therefore might not work with options that are specific to it. If so, you may want to change it to use a more conventional chain-loading boot approach. Alternatively, if you're currently chain-loading, you could give a direct boot from GRUB2 a try, although my impression is that OSx86 boot loaders do more tweaking of the configuration than GRUB2 can do, so my suspicion is that a direct GRUB2 boot would be unlikely to improve matters. It's worth a try, though.
If you could post the section of your grub.cfg file that relates to OS X (just the OS X section; chances are nothing else is relevant), then I (or somebody else) might be able to offer more specific advice.
Another option, should you find that you just can't get GRUB2 to work for you, is to replace GRUB2 with GRUB Legacy. Chances are you'd need to remove a package called grub2 and install one called grub. You'd then need to create a completely new GRUB configuration (using /boot/grub/menu.lst) and run grub-install to install it. I'm not running Ubuntu 9.10 beta myself, though, so I'm not 100% sure of these package selections or what Ubuntu-specific caveats might exist in such a switch. It's conceivable your system wouldn't boot at all when you're done.
Yet another option might be to reconfigure GRUB2 to run as a secondary boot loader, using an OSx86 boot loader (Boot Think, Chameleon, PC_EFI, etc.) as your primary boot loader. This would require re-installing both boot loaders, and there's a high probability that you'd run into problems you'd need to debug along the way.
