This is a list of the currently assigned menu-item numbers. If you need a number for an official udeb, don't just grab one, but ask on debian-boot@lists.debian.org first.
Please do not allocate numbers with too high a granularity. If you have two packages, A and B, and A depends on B, then main-menu will always put A after B. There is no need to assign different numbers to the two; one number will do and will be more flexible, and help avoid BASIC line number disease. In other words, having a bunch of menu items with the same number is a feature, not a bug. Use different menu items only to break ties and do gross positioning in the menu.
For the Squeeze release these numbers were multiplied by 100 from what they were before to give more flexibility.
0 -> 1000 Reserved
1000 -> 3000 Preparation and installer setup
1000 localechooser (!s390)
1100 load-media
1200 kbd-chooser
console-setup
1300 iso-scan
cdrom-detect
1400 file-preseed
1500 driver-injection-disk-detect
1600 load-iso
load-cdrom
1700 ethdetect
ppp-udeb
s390-netdevice (s390)
1800 netcfg
netcfg-static
2100 network-preseed
network-console
2110 localechooser (s390)
2300 choose-mirror
download-installer
2400 user-setup
2500 lowmem
2600 clock-setup
3000 -> 4000 Disk detection etc.
3500 disk-detect (!s390) 3700 s390-dasd (s390) 3900 rescue-mode
4000 -> 5000 Disk preparation
4200 partman 4400 lvmcfg 4500 mdcfg
6000 -> 8000 Base installation and completion of install
6500 base-installer
7000 apt-setup
pkgsel
7300 zipl-installer (s390)
flash-kernel-installer (arm*)
7400 grub-installer
7700 nobootloader
7800 finish-install
8000 -> 9000 Reserved
90000+ Misc
93000 cdebconf-priority
94000 cdrom-checker
save-logs
95000 di-utils-shell
96000 eject-udeb
99000 di-utils-exit-installer
99500 di-utils-reboot
99999 dummy menu item for components that should be listed in anna,
but not in the main menu