udev "forgets" to read rules on boot

Binary package hint: udev

I have written a custom udev rule which will assign group "users" to any /dev/tty* devices (USB serial converters). I need this for Palm Pilot syncing over USB cradle.

According to the README:
If the ordering of files in this directory are not important to you,
it's recommended that you simply name your files "descriptive-name.rules"
such that they are processed AFTER all numbered rules in both this
directory and /lib/udev/rules.d and thus override anything set there.

I have placed the rule in /etc/udev/rules.d/ttyusb.rules

Sometimes udev seems to "forget" this rule on boot, when plugging in the device /dev/ttyUSB0 is group "root". Doing "touch /etc/udev/rules.d/ttyusb.rules" an re-plugging restores group "users". As I said, this is only sometimes and not reproducible. For the moment, I placed the "touch..." command in /etc/rc.local, which seems to help.

What can I do to debug this further?

ProblemType: Bug
DistroRelease: Ubuntu 10.04
Package: udev 151-12.2 [modified: lib/udev/rules.d/50-udev-default.rules lib/udev/rules.d/40-pilot-links.rules]
ProcVersionSignature: Ubuntu 2.6.32-26.48-generic 2.6.32.24+drm33.11
Uname: Linux 2.6.32-26-generic i686
Architecture: i386
CustomUdevRuleFiles: ttyusb.rules usb-audio.rules
Date: Thu Dec 23 09:18:13 2010
MachineType: Hewlett-Packard 315eu
ProcCmdLine: BOOT_IMAGE=/vmlinuz-2.6.32-26-generic root=/dev/mapper/vg1-root ro quiet splash
ProcEnviron:
 PATH=(custom, no user)
 LANG=de_DE.utf8
 SHELL=/bin/bash
SourcePackage: udev
dmi.bios.date: 04/23/2010
dmi.bios.vendor: American Megatrends Inc.
dmi.bios.version: 6.01
dmi.board.name: 2AAF
dmi.board.vendor: FOXCONN
dmi.board.version: 1.00
dmi.chassis.asset.tag: CZC0334WG9
dmi.chassis.type: 3
dmi.chassis.vendor: Hewlett-Packard
dmi.modalias: dmi:bvnAmericanMegatrendsInc.:bvr6.01:bd04/23/2010:svnHewlett-Packard:pn315eu:pvr:rvnFOXCONN:rn2AAF:rvr1.00:cvnHewlett-Packard:ct3:cvr:
dmi.product.name: 315eu
dmi.sys.vendor: Hewlett-Packard