The authors look at implications of a varying \Lambda(t) and G(t). I do have some comments that are not ment as criticism to this paper (which has a different spirit), but rather as general remarks on the topic:
In a general covariant theory, there is no quantity \Lambda(t): general covariance implies that it must be a function of space time, hence \Lambda(t) > \phi(x) and we are back at a scalar field theory.
Secondly, a change of Newton's constant G(t) and hence the Planck mass can be absorbed by a Weyl scaling. Of course, it is a matter of taste and convenience which frame might be more suitable. In principle, however, one can get rid of G(t) and will introduce an additional scalar field plus some matterfield couplings.
[astroph/0507110] Dynamical dark energy versus variable cosmological constant
Authors:  Joan Sola, Hrvoje Stefancic 
Abstract:  One of the main aims in the next generation of precision cosmology experiments will be an accurate determination of the equation of state (EOS) for the dark energy (DE). If the latter is dynamical, the resulting barotropic index should exhibit a nontrivial evolution with the redshift. Usually this is interpreted as a sign that the mechanism responsible for the DE is related to some dynamical scalar field, and in some cases this field may behave noncanonically (phantom field). Present observations seem to favor an evolving DE with a potential phantom phase near our time. In the literature there is a plethora of dynamical models trying to describe this behavior. Here we show that the simplest option, namely a model with a variable cosmological term, Lambda=Lambda(t), leads in general to a nontrivial effective EOS which may naturally account for these data features. We also show how this effect is modulated by a variable Newton's constant G=G(t). 
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[astroph/0507110] Dynamical dark energy versus variable cos
I am not sure that a theory with variable G and Lambda is equivalent to a scalar coupling, modulo a Weyl rescaling. If G and Lambda are determined by some renormalization group trajectory, as in the quoted paper by Sola, it is not possible, in general, to describe their evolution of from an action principle. In other words, the equation of motion for the "scalar field" is not necessarely also a solution of the renormalization group equations. Possible solutions to this problem are presented in Phys.Rev.D69:104022,2004, and in Class.Quant.Grav.21:50055016,2004. On the other hand, one can imagine that a variable G and Lambda would act as a source term in the energy balance equation, and we are simply describing an exchange of energy between the dark sector and the matter sector.