Jared Spool has written an interesting continuation to the discussion he started at IA Summit 08 in the IxDA mailing list. He proposes that Activity-centered design is a subset of User-centered design. Where the former is concerned with the activity/tasks necessary to interact with the system, the latter is more aware of the context of use, goals, and motivations of the user of the system.
I created a quick diagram to wrap my head around the spectrum of methodological approaches Jared speaks to:

I see the larger bubbles requiring more process to complete while the smaller bubbles have lower and lower user involvement.
And to define each methodology I’ll borrow from Jared directly:
“0) Unintended Design: The design that results from teams that don’t pay any attention to design. This is the true rubber-band-and-spit approach to creating things. Everything ends up with a design, but not every design is intentional. Some very lucky teams end up with successful unintended design, but the odds are against this.
1) Self Design: The design that results from teams that design purely for themselves. (This happens more with single-person teams than multiple-person teams.) This design approach has better odds of success than Unintended Design, but not by much (unless the designer is the only user, such as when a bachelor arranges the contents in their kitchen cabinets). This design approach is only informed by the team members own use of the design.
2) Genius Design: The design that results from teams that use their experience at creating designs for others, without doing research. This starts with Self Design, but extends to role playing and consideration of users who are not quite like themselves. This design approach is informed by previous experience the design team has with similar work. (For example, a team that creates shopping cart systems for many clients can reduce their research efforts with each subsequent implementation, assuming the systems are pretty much the same each time. Eventually, they could create very successful without further research, since they’d basically “seen it all”.)
3) Activity-Centered Design (ACD): The design that results from teams that only research the activities. Because research is part of the design process, it extends beyond Genius Design (which solely is based on the team’s experience). This is necessary when the activities are new or foreign to the team. (For example, a team developing an application for consolidating personal finances when they’ve never thought about personal finances in any of their previous projects.) Activity-based research techniques, such as workflow diagrams and task-based usability tests would come in very handy to help inform the teams using this approach.
4) User-Centered Design (UCD): The design that results from teams that look beyond just the activities, to the goals, needs, and contexts of the users. Because usage is all about activity, this approach needs to have the activity at its core. (Early UCD definitions always included an essential “task analysis” phase — something that’s disappeared from the lexicon, but is still essential to this design. Task analysis is, as far as I can tell, research about activities, and thus the core research component of ACD.) This design approach is informed by techniques such as field research (ethnographic techniques) and persona creation, which help the team to see contextual items and goals.”