Currently a cell is considered the basic unit of life.  Viruses themselves are not cellular, but have to take control of host cells to harness replication. 

The definition of life, like the definition of intelligence, is ambiguous and vague. Various scientists will choose a definition for specific purposes, so some scientists consider virii alive, while others do not. There is no hard consensus, except that when it is taught in school, they generally stick with cellular life to make the study of biology simpler. Cellular life has: cells, metabolism, growth, reproduction, etc. You know the drill.

But there are various definitions of life which have interesting uses. For example, one very good definition of life is anything that replicates with variation and selection. Basically, anything that evolves. While this blurs the edges of science (biology is not so cut-and-dried under this definition), it is a very useful definition because it gives insight into non-biological life. For instance, alien life. If we were to go to an alien planet, how would we know if it has existing life on it or not? The life might not be carbon-based, it might not be cellular, etc. etc. But if it is true life, then it will definitely evolve, and we can detect that by detecting replication with variation and selection.

Using the same definition, we can see that virii are a form of life, whereas prions are not (there is no variation). Some other instances of this kind of life: Memes, technology (designs), genetic algorithms,
computer viruses/worms, etc.

After such an insight, you can start getting into much more 'out there' examples like stars and planets. Stars are a form of life in this sense because each generation of stars is produced from the debris of the previous generations of stars. There's variation in the elemental composition of the stars, which is inherited from the previous stars' supernovae. There's even selection because some stars are more stable than others, so we see an actual evolution of stars. Of course, there have only been about 3 generations of stars so far, so they are not very complex yet, but each generation is undeniably more complex than the last.

Planets can also be considered a form of life, in the sense that if we colonize other planets, they will become like Earth (replication), but there will be differences (variation), and some of those planets will die off due to the variations (selection).

Even things like corporations can be considered a form of life in this definition, because corporations can splinter into related corporations, each with different business plan and skills, and those qualities will influence whether the corporation stays in business or not. The 'invisible hand' of Adams is actually just evolution!

Finally, and this is where it ties into the RRS, you can consider religions and holy books as a form of life. This is above the idea of simple memes, just like a cell is above the idea of genes. Religions splinter with variation (just look at Christianity) and those variations influence the survival of the religion. There was some group of Christians (Quakers?) that basically died off because one of the tenets was against having sex, even for making babies. Clearly that was a deleterious mutation.

Why would you want to consider religion/books a form of life? Because it helps you to understand them and make predictions about them. For instance, I predict that there will be a dramatic upsurge in the number of religious sects, due to the internet. The internet is like a new niche. You can imagine it as the transition from ocean life to dry land. When the first vertebrate found a successful niche on land, there was a dramatic adaptive radiation that eventually led to dinosaurs and, later, mammals. That's just what happens when life finds a new niche that is wide open.

So, the real strength of this way of viewing life is that it allows you to make powerful predictions by analogy with other similar life-forms. E.g. Is a new trend just a fad, or will it change the world? You can get a much better picture if you understand evolution at this level. For instance, is YouTube just a fad, or will it (video presence on the web) change the world? I predict it will change the world, and I base this prediction on my understanding of ecology and the evolution of cooperation, as well as an evolutionary understanding of human nature. It allows similar people who are scattered far and wide (like atheists) to form a cohesive group which would not otherwise be possible. When groups form, it allows mutual cooperation to occur; isolated individuals end up being exploited by groups. The video aspect allows faster communication of empathy, compared to text, or even audio podcasts. With empathy, the newly formed group becomes much more cohesive and cooperative than without it. Such scattered 'groups' can achieve more than they could if they were not connected. This is how it will change the world and not end up just a fad. 

That's just one example. The number of insights I get from this kind of thinking over the last year or so has been mind-blowing to me. 

I think the generally accepted definitions of life that I've been exposed to are far too limitting to be valid. Anything and everything within it is soley based on one life forms experiences over an insignificant amount of time on a tiny insignificant rock in a remote corner of a shadow of a incomprehensible abyss.