Liberty has a page on house arrest at the moment, with an online petition (to be presented this week). At 5:30pm on thursday, liberty will be holding a vigil in parliament square as the bill goes to the Lords.
One can join Liberty here.
Additional: I've just seen a copy of Boris' address to parliament. Excellent stuff. It's at times like this that we need 'Have I got news' on the air - with Paul Merton in full attack mode (he wouldn't support the bill... surely?) Coupled with Boris by his side and the possibilities are boggling! If we can't get a Lib Dem chap then.... Boris for PM!