I am in L1b Visa stamped till March 26 2011
My I94 is stamped till 2013.
When I applied for visa extension, my company told that I can continue to work till 2013 and I will be provided with a new I 129 valid till 2013.
Is this a legal approach? Is the new I129 that they are referring to, approved by USCIS?
I am planning to go back to My country on April 2011 for my marriage. That time my I 94 would have been taken out. Then how can I get back to US? Will I be able to get the new visa stamped with the newly issuing I 129s? Isthere any chance of rejection there?
Please help me.

Your I-94 is for the period on the I-797 approval notice. The visa is probably limited per the FAM reciprocity schedule for your home country.

So, you can work until 2013 as per your I-797 and I-94. If you need to travel after March 2011, you apply for a new visa with the same I-797 you used previously.

Confirm this with the company immigration attorney.

Congratulations on your marriage.