...the increase in temperature in the 20th century is likely to have been the largest of any century during the past 1,000 years...

The IPCC TAR SPM said:

New analyses of proxy data for the Northern Hemisphere indicate that the increase in temperature in the 20th century is likely to have been the largest of any century during the past 1,000 years. It is also likely that, in the Northern Hemisphere, the 1990s was the warmest decade and 1998 the warmest year (Figure 1b).

naturally, this was a red rag to the septics. Its was based, as it says, on Jones et al., 1998, Mann et al., 1998, 1999 and Briffa, 2000. How does this stack up, when compared against more recent reconstructions?

The answer is, extremely well. There is a nice pic on wikipedia (and I've inlined it above) which shows 10 different reconstructions. Looking at this pic we see that

1. the increase in temperature in the 20th century is likely to have been the largest of any century during the past 1,000 years is true for all of them (and for those that go back further, it looks true of the previous 1000 years too)
   
2. It is also likely that, in the Northern Hemisphere, the 1990s was the warmest decade is also true
   
3. and 1998 the warmest year also true (it happens not to have been exceeded since 1998, and most likely not earlier)
   

Another interesting feature of those graphs is that they all pretty much agree on the temperature at the peak of the MWP as being about 0 on a scale that has "now" as about 0.4 (with the caveat of resolution). The major differences are on the depth of the LIA - about 1600, there is little agreement.

Which is why (you just knew I was coming to this didn't you) all the ho-ha over MBH98 is interesting statistically (perhaps) but irrelevant scientifically. It also ought to be irrelevant politically.

To debunk another couple of myths: idiots sometimes assert that the Hockey Stick (ie, MBH98/99) underpins Kyoto. This is nonsense for any number of reasons, but the most obvious is that Kyoto was negotiated in 1997 and you need to believe in reverse causality to think that a 1998 paper underpinned it.

Another one is pointed out by this RC post: The famous conclusion of the IPCC, “The balance of evidence suggests that there is a discernible human influence on global climate”, does not depend on any reconstruction for the past millennium. It depends on a detailed analysis of 20th Century data. In fact, this conclusion is from the 1995 IPCC report, and thus predates the existence of quantitative proxy reconstructions like the “hockey stick”.

Septics focus on MBH98 because they think they have something to say about it. People, naturally, respond. All this tends to give a rather unbalanced view of where the science is. The palaeo reconstructions are important, but only one strand. There is lots and lots of D+A (DAtDA?) that people tend not to discuss much, rather in the same way that few people discuss detailed solutions of GR much: its just too complex for anyone except specialists.

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Note, BTW, that "likely" is defined thus: In this Summary for Policymakers and in the Technical Summary, the following words have been used where appropriate to indicate judgmental estimates of confidence: virtually certain (greater than 99% chance that a result is true); very likely (90-99% chance); likely (66-90% chance); medium likelihood (33-66% chance); unlikely (10-33% chance); very unlikely (1-10% chance); exceptionally unlikely (less than 1% chance). The reader is referred to individual chapters for more details.