Ideally what should happen is that Ubuntu and Debian remain compatible enough that they can share the same repositories.
If Ubuntu is based on Testing or is a snapshot of Debian unstable then it should be fine not to actually provide any software other then what is in Ubuntu main and then just have people pull directly from Debian repositories.
If Debian and Ubuntu get their 'stable' releases syncronized then that should make it even more possible.
It's difficult situation both ways.
Ubuntu was created, essentially, as a response of Debian's inability to get software out on time and create a user-friendly default desktop install. (If this is not true then Ubuntu would of never reached the level of popularity that is today.. people would simply be using Debian instead)
I remember back in the day when a bunch of companies like Progeny and Xandros and all that tried to develop a "standard debian base" and all that so that they could share packages between them. But all those efforts failed because of Debian's failures and limitations. Debian was just unable to produce software releases on a dependable and timely manner.
If Debian developers could be made aware of the value and advantages to time based releases AND Ubuntu can gain the discipline in packaging to remain compatible with Debian then big benefits both ways can be realized.
That way Debian gains user-friendly desktop and Ubuntu gains credibility as a really useful OS in the business/server world. Debian's big advantage is the quality of packages and the amount of work that goes into making sure that everything is consistent, tested, and compatible. If, for example, your doing something big like setting up a Kerberos/LDAP system for Linux desktops then the packages provided by Debian will actually work for that, unlike most distros that don't get that stuff tested well or depend on proprietary products. It's obvious from the amount of details and little compatibility configurations and tools that people actually use Debian for that stuff. Ubuntu needs that sort of thing to be taken seriously as a desktop operating system for businesses.
I do NOT see any reason, other then people's hangups and personal politics, why (with work and coordination) the Debian Tasksel that you won't be able to pick 'Ubuntu Desktop' as a installer option. I also think that instead of having the 'universe' and 'multiverse' users could just point at 'deb http://ftp.debian.org/' for their extra package mirrors.
I think that would be a wonderful thing to shoot for.