Working on an old H1B petition of Previous employer

[kayess]

I am currently working for an employer say 'B', whose H1B petition and I-94
are valid till Nov, 2010. I got my current I-94 while traveling back to US at Port Of Entry based on I-797 of my current employer 'B'.

Now can I go back and work for my previous employer say 'A' whose H1B petition is still valid till August, 2010. Do I have to inform USCIS in order to do that.

If I go back and start working for my previous employer 'A' without informing USCIS,
Will I have a problem when I apply for my H1B extension in August, 2010 through my previous employer 'A'.

Guys please advise