HP's idea of grid computing is unique. It is of a reconfigurable interlinked array of Smart cells, meaning storage plus intelligence. A white paper is available to describe it here. HP has a RISS or Reference Information Storage System as a first example of this.
Our columnist Hamish MacArthur has written "The concept of utility is a step in the direction of managed environments, where users obtain better utilisation of their computing resources. The delivery of this concept will be a mixture of new storage, server and networking technologies that offer an assured delivery of better system utilisation. As businesses expect round the clock computing services and access to data, the systems will be highly resilient. Grid storage is one of the key architectural developments for the delivery of utility storage."
What HP is offering by way of yesterday's public utility announcement is a $/hour/use way of paying for HP computing resources. This, in principle, is exactly similar to what Sun is offering. Sun calls its offering the Sun Grid. In HP's view the Sun Grid is obviously not a grid; it's a utility-like charging scheme and HP has copied the concept.
But neither Sun nor HP is offering what has been described as utility computing, described by Hamish above. It's probably better described as a utility-like charging scheme.
Grid computing, including grid storage, is not equivalent to utility computing. A grid computing scheme may be a way of delivering utility computing, and it may be charged for in a utility-like way, a $/hour/resource use model. But utility computing is not the same thing as a utility-like billing for IT resources.