Ubiquity completely freezes when activating the forward button on the partitioning step, when the GNOME accessibility framework is enabled, and the orca screen reader is being used. This seems to occur regardless of whether modifications to the partition scheme have been made or not.
Steps to reproduce:
1. Boot a copy of the beta release desktop CD.
2. Enable the GNOME accessibility framework in System -> Preferences -> Assistive Technology Preferences. Check the box labeled "Enable accessibility technologies." No other boxes need to be checked.
3. Close the window, and log out/in again.
4. Once logged back in, bring up a terminal, and run "sudo orca". This will ask you a few questions. You can answer these however you like. You will then be returned to a prompt. Run orca again with the same command. Orca will run, printing its output to the terminal.
5. Launch the installer, and fill in options as required, making sure to select the manually partition option. The partitioner appears, and the installer should freeze when you activate the forward button from this step, if you haven't made any partition changes. The installer should also freeze once any partitioner changes have been saved.
I haven't been able to reproduce this here. Would it be possible to get an 'strace -f -tt -s 1024' of ubiquity to try to see what it's doing? To reduce the size of the trace, you might want to attach strace after selecting "Manually edit partition table" but before pressing Forward on that page.