The Team (at both realclimate and NOAA) have stated in the strongest possible terms that the Holocene Optimum was restricted to summer in the NH extratropics. Reviewing their positions, realclimate here
The [Holocene Optimum] is a somewhat outdated term used to refer to a sub-interval of the Holocene period from 5000-7000 years ago during which it was once thought that the earth was warmer than today. We now know that conditions at this time were probably warmer than today, but only in summer and only in the extratropics of the Northern Hemisphere.
NOAA (with a very recent update in Nov 2006) stated:
In summary, the mid-Holocene, roughly 6,000 years ago, was generally warmer than today, but only in summer and only in the northern hemisphere.
On a previous occasion, I discussed recent findings of Newton et al with convincing evidence of an MWP in the Pacific Warm Pool (as well as related evidence in Stott’s webpage (the series is identifiable as MD81 although not specifically identified on the webpage.) Here is an important graphic from Stott et al (Nature 2004) showing the location of 3 cores in the PAcific Warm Pool (top) with Mg/Ca temperature series and dO18 salinity series below, about which Stott says the following:
Note how the d18O values become progressively lighter through the Holocene whereas the temperature show a maximum in the early Holocene and decline (in the western Pacific) towards the present. The trend in d18O during the Holocene reflects the progressive freshening of the western tropical Pacific during the mid to late Holocene.
From Stott et al 2004. Blue – Mg/Ca proxy for temperature; red – dO18
Reference: Stott etl al, 2004. Decline of surface temperature and salinity in the western tropical Pacific Ocean in the Holocene epoch, Nature, 431, 56.