I was getting this weird error "No DHCPOFFERS received. No working leases in persistent database - sleeping.", when trying to connect to internet via WIFI in Ubuntu.
After enough googling to no avail, and by sheer serendipity I solved this. This worked for me; may be it helps you too.
Argh, I missed the point. So here is what you can try if you are stil having this issue.
Step 1: Go to, System -> Preferences -> Network Connections
Step 2: Select Wireless tab
Step 3: Click "Edit" the wifi connection profile you have created
Step 4: There are two connection modes - Ad hoc and Infrastructure. Choose the "Infrastructure" mode
Step 5: Save and try connecting again. You are done...!