To clarify the matter, what H-1Buser has said is essentially correct. You LC (once approved) is valid for life, however to get a GC based on that particular LC, first you need the same employer to file your I-140, and second you need to be working for that employer when you do get your GC (unless ofcourse your I485 is pending for more than 180 days and you use portability).
You LC and I140 are employer specific, so if you are not working for that employer he may withdraw your LC, OR he may not want to file I140 for you.
So you can still apply for H-1 B extension based on your first employer's LC petition (if not withdrawn) and all that good stuff, but in order to get a green card thru that petition, you will have to get that employer to file 140 on your behalf which might not be feasible in all cases. That might be what H-1buser went thru.
Afterall your goal is to get the GC, not just extend your H-1. So if your employer doesn't withdraw your LC, you shud get H-1 extended based on that, and then work on your second LC petition to get the GC.
In today's scenario (pre HR 2215), you need have an approved LC and I140 filed to get the extension.