In most states - its a 30-year rolling cut-off for 'Historic' rego. Its also a *shitload* cheaper - here in ACT, the full cost of rego for a bike with a historic plate, is under $100 a year.
You do have to belong to a club that is affiliated with the 'controlling body' in your state. Here, it is the ACT Council of Motorsport Clubs. Only a certain number of clubs are allowed to utilise the historic rego scheme, so it will pay to check.
For your XL500

, the best bet is to join the VJMC. These guys have all the info you need for historic rego. But you won't qualify for this type of rego until 2013.
Once your bike has an historic plate, there are certain restrictions. Don't ride it to work every day, if the bronze see this, you will lose your rego and they will have more ammo to have this scheme removed entirely. There are nominal limits to annual mileage as well. Its about 3000km pa down here, but this is a nominal limit - some of the annual club runs can chew up your mileage amount in 1 trip.....
Basically - if you only ride the bike every weekend, you will be sweet. There are requirements to carry a log book, and you are obliged to record each trip - but trips can be for anything from collecting parts, to test-rides after a tune-up.....

- down here, its actually written into the rules that a trip around to a friends house to show them your bike, is a legitimate reason for riding !!
I know a lot of the guys in the Victorian Z Owners club are all running club (historic) rego plates these days, as their old Zeds are not used as daily runners. Its definitely worth checking out the Vic branch of VJMC for more info, but you do have a few years to wait until you can use club plates.
