If you only regestered your interest then you can pre-order anytime once BT have announced your exchanges RFS (ready for service) date.
If you have pre-ordered with Plus.net they will automatically place an ADSL order with BT on your RFS date.
Check you ADSL tracker - click on connection settings then ADSL tracker (you may need to enter your Plus.net user name and password), from here you can check your ADSL progress. I pre-ordered ADSl with Plus.net back in October when my exchange RFS date was announced by BT.
My exchange became RFS on Wed 11th - everything was then done automatically. Plus.net do an initial check to make sure your exchange is enabled, they then take your activation fee (if applicable) and place the ADSL order with BT. My provisional activation date is Thursday 19th Feb. Speaking to other people in my area they all now have this as their provisional date - hopefully it will be done sooner though.
If you have ordered DSL connect you will not be able to see anything in your ADSL tracker.
As far as I'm aware all exchange RFS dates are on a Wednesday so I dont think anything will happen until Wednesday.