We have heard that when you purchase a VMware vSphere license, you get a free SUSE Linux Enterprise Server. Is this offer still valid?

No, this offer was discontinued by VMware on June 25 2014, Until then, a free activation code for the SUSeE Linux Enterprise Server 11 was obtained with the purchase of VMware vSphere Standard, vSphere Enterprise, or vSphere Enterprise +. This offer was available under the name SLES for VMware and contained a subscription for free updates and patches, but no support . However, existing installations can still be used free of charge.

Sources:

Whitepaper_SLES for VMware End of Availability, Stand August 2015

Official VMware SLES Website, as of 04-08-2016 (https://www.vmware.com/de/products/sles-for-vmware.html)