I strongly agree with Jay H. WordPress offers a great foundation, and easy interface for clients. If you're already having them use WP, why not have it all-in-one.

Don't know if you all have seen this: http://templatic.com/news/gtd-%E2%80%93-private-blog-theme-for-teams-to-collaborate but they've taken Automattic's awesome P2 theme (which Automattic says is changing the way they work and collaborate) and made it look like basecamp, plus added some basecamp-like attachment & notification features.

I've used basecamp quite a bit. I remain thoroughly unimpressed with almost every aspect of the software. If somebody would make a plugin (or update WP-Project) to allow all posts to be tagged by project, and some kind of simple task-list feature that could be displayed on pages & posts, and allow each task to be assigned to a user, I'd quickly get all my basecamp-using clients to move to that.

In my experience, without true whiteboard functionality (like live drawing, etc), basecamp mostly gets used for 2 things: message conversations, and task lists/assignment. Thus if a plugin were to add task lists/assignment capabilities, especially with project tagging, P2 would do everything that I see basecamp used for, and do it much better.

So I guess I'm saying that I, for one, would love to see WP-Project updated and expanded.