Does the heating of the oceans due to greenhouse effect negate the acidification due to increased CO2 levels?
This is a very complex issue, and therefore I'm not entirely convinced I am qualified to answer that, considering how little I understand chemistry, but I'll give it a try.

The Flux of CO2 can be expressed as follows (see e. g. Wanninkhof et al. 2009):

$$F = kK_0(p_{\mathrm{CO}_2sea} - p_{\mathrm{CO}_2air})$$

By convention a negative $F$ means a flux from the atmosphere to the ocean.
$k$ is the transfert velocity and $K_0$ is the solubility which indeed decreases with temperature, as you noted.
$p_{\mathrm{CO}_2sea}$ and $p_{\mathrm{CO}_2air}$ are respectively the partial pressure of CO2 in the sea and in the atmosphere.
Now, with $T$ increasing, $K_0$ (and also $k$ I believe)  decreases, but, as long as the partial pressure of CO2 is larger in the atmosphere than in the ocean there still is a flux toward the ocean.  

So worst case scenario, increasing temperature slows down the absorption of CO2 by the ocean but does not negate it. However of course, if the temperature increase to begin with, it is because $p_{\mathrm{CO}_2air}$ is increasing, so this is a bit of a moot point anyway.  

As far as the ocean buffer goes, we're talking here of an uptake of ca. 2 Pg (i.e. $2\times10^{15}$g) of carbon dioxide per year (the preindustrial uptake being ca. 0.5 Pg according to Takahashi et al. 2009), which is too much to be buffered. For comparison, phytoplankton net primary productivity is ca. 50 Pg of fixed carbon per year (Falkowski et al. 1998).

Falkowski, P. G., Barber, R. T., Smetacek, V., 1998. Biogeochemical Controls and Feedbacks on Ocean Primary Production. Science, 281: 200-207.
Takahashi, T., et al. 2009. Climatological mean and decadal change in surface ocean pCO2, and net sea–air CO2 ﬂux over the global oceans. Deep Sea Research II, 56: 554-577.
Wanninkhof, R., et al. 2009. Advances in Quantifying
Air-Sea Gas Exchange and Environmental Forcing. Annual Review of Marine Science, 1: 213-244.