Ok, I successfully installed BOINC 4.05 on my computer, replacing 3.02. Now I'm seeing the following messages from Predictor@Home:

Predictor@home - 2004-09-09 09:22:36 - Sending request to scheduler: http://predictor.scripps.edu/predictor_cgi/cgi
Predictor@home - 2004-09-09 09:22:47 - Scheduler RPC to http://predictor.scripps.edu/predictor_cgi/cgi succeeded
Predictor@home - 2004-09-09 09:22:47 - Message from server: To participate in this project, you must use major version 3 of the BOINC core client. Your core client is major version 4.
Predictor@home - 2004-09-09 09:22:47 - No work from project
Predictor@home - 2004-09-09 09:22:47 - Deferring communication with project for 1 days, 0 hours, 0 minutes, and 0 seconds

I had made the upgrade to get rid of a similar, but opposite-sensed, set of messages I was getting from SETI@Home - basically, it said I had to be running version 4 of the client to participate instead of version 3.

Wouldn't it be easier to make new clients compatible with the previous major update, instead of "throwing the baby out with the bathwater" by completely rewriting the clients with no regard to past server work? And if it is not, what is the target lag time for the indivivual project servers to be running the same core code?

Those of us participating in multiple projects are going to tend to pause at the very idea of adding more projects if these type of messages are going to become frequent. A bit senseless for a person to upgrade to get rid of a set of messages from one project that says the client software is too old, only to generate a set of messages from another project that says the client is too new!