I saw a posting on SO and applied via their website. Same scenario as others have stated:
1. Email hello - thanks for applying
2. Phone screen w/ tech recruiter (mostly general intro's to each other and talk about BT)
3. Take home assignment - a really good practical to flex your skills.
4. A phone screen with two other engineers to chat about past performances, outcomes and challenges. Be ready to talk about technical hurdles you've overcome and what that's done for your own skill set.
5. On-site interview - long day, I think 4 sessions in total with 8 people.
Overall, a really wonderful experience. The inside recruiter was prompt with replies, though there was a small gap when it came time to schedule the onsite (I later found out that headcount was still being finalized). The people I met with in the interview were kind, professional, ego-free (mostly) and smart. It made me want to work there even more.