The "-z3" option should be used with cvs and the "-z" option should be used with rsync to compress data sent over the network. For modem users this can reduce "cvs -dP update" time to somewhere around 20% of the time it takes uncompressed. -z3 and -z should not use much in the way of cpu time on the servers and is a big win in the bandwidth department.
http://bugs.gentoo.org/show_bug.cgi?id=5965 also deals with improving rsync
Fixed for CVS (rsync already done) in 2.0.35. Thanks :)