Since this evening, parallel booting is the default in Debian/unstable for machines where dependency based boot sequencing is enabled. Apparently the testing of concurrent booting has been wider than expected, if I am to believe the input on debian-devel@, and I concluded to move forward with the feature now, to give us some time to detect any remaining problems before Squeeze is frozen. If serious problems are detected, it is simple to change the default back to sequential boot.
More information about dependency based boot sequencing is available from the Debian wiki. It is currently possible to disable parallel booting by adding this line to /etc/default/rcS:CONCURRENCY=none
If you report any problems with dependencies in init.d scripts to the BTS, please usertag the report to get it to show up at the list of usertagged bugs related to this.