A separate partition for /home has been proposed for a long time in the forums. It implies some risks, though, so based on disk size Ubiquity should estimate the amount of space that should be left for / or whether a separate partition should be made at all. Then...
-The first time an user installs Ubuntu, he is given the option to set a separate /home. This option is selected by default, with a size for each partition based on a sane guess:
http://img155.imageshack.us/img155/7958/firstinstallaro2.png
-Of course, he can just choose not to set a separate /home. This option will be selected by default if the results of the system test suggest that's the best thing to do.
http://img186.imageshack.us/img186/6498/firstinstallbfs6.png
-Manual install is also possible. Selecting it greys out everything related to separate /home, since it's implied that the user doesn't want to be guided.
http://img177.imageshack.us/img177/7976/firstinstallcvc2.png
-If the user set a separate /home, the next time he installs Ubuntu a new option appears and is selected by default, prompting to use the existing /home partition. All other options are still available, though.
http://img155.imageshack.us/img155/9034/secondinstalliq1.png
[....]
Developer comments
I have always used a separate /home as well. Keeping /home after reinstallations is one major reason which has been mitigated by ubiquity preserving /home now (I didn't test that yet, though).
Either way ubiquity (the Live CD installer) should point this out clearly.
The other reason is that I want to use it from multiple Linux installations, but that's mostly a geek use case.
I have no idea what size / and /home should have by default,
I always use 6 GB for / and the rest for /home, but if someone wants to use huge databases, that'll break.
Thus I think we should stick to our current partitioning and rather improve the UI for keeping /home. There is always manual partitioning for people who actually care.