VMware released ESXi 5.1 Update 1 at April 25. The update contains mainly bug fixes and some enhancements. No new features.
See the release notes here.
Storage enhancements of Update 1 are discussed in this great blogpost by Cormac Hogan.
Interesting adjustments are an increased maximum heap size. This allows to have an ESXi host a much larger amount of open VMDK files on VMFS volumes. More info on heapsize and how to increase it for vSphere 5.0 can be read in my earlier posting.
Also it is again possible to rename VMDK files while performing a storage vMotion. Frank Denneman has written a blogposting about this.
Before performing the upgrade, make sure other VMware components are compatible with Update 1. Viktor van den Berg has a short posting about this here.
Also mind that upgrading VMware solutions needs to be done in a certain sequence. VMware has details in a post titled Update sequence for vSphere 5.1 Update 1 and its compatible VMware products (2037630)