.Hi,
I work for company A and my H1B is Effective from last year. I wan't to switch to company B.

Suppose the company B files H1B petition (for transfer, same process as for new one) with starting date of Nov 1 2007. If this petition is aoproved,

1. Is it mandatory to join Company B? Or can I continue working with company A ignoring new H1b approval?
2. If I must have to join Company B, shall it be immediate or is there any specific period whithin which I have to join?

thanks
trader

1. Is it mandatory to join Company B? Or can I continue working with company A ignoring new H1b approval?
You cannot continue working for Company A as of the H1B transfer start date (NOv 1st in ur case). The new H1B will be for Company B so, u will have to join them and not any other company.

2. If I must have to join Company B, shall it be immediate or is there any specific period whithin which I have to join?
You can start working for company B as soon as you get ur H1B transfer start date kicks in. If the company B is willing to give u some time in between ur transfer date and ur actual start date then u r can take that break too.. It complete depends on how flexible are company B.

NOTE: I am not an immigration expert. Please consult your attorney for details.

.Anybody else with more insights?

Google ur questions. There are more reliable answers there. look under h1b transfers.