I'm not going to suggest - for one minute - that I'm right here...just going to tell you my PERSONAL experience.
I was employed in the USA on an H1B. The company then made us all redundant. Enquireies direct with Immigration told us that the visa is only a document that allows you to cross the International boundary into the USA. On any day of entry to the USA on the valid visa, the INS man at the airport will enter a date into your passport - written on the stub of a form I-94 (which is then stapled into the passport).
INS told me that you have the legal authority to remain in the country until that expiration date - whether working or not - and that you are entitled to seek other employment WITHIN THE SAME FIELD OF WORK with any other employer. The new employer must apply for a new H1B visa only if they intend to keep you after the date of expiration of the I-94.
The H1B approval (when granted) is for seven years from the date of application - not date of issue. The H1B's however are issued in three years blocks - so you can renew twice - although the last one would only be valied for one year.
I say agin - this is all as told to me by the man at INS (in Miami).
If the information does prove to be incorrect - blame INS for not knowing their job.