This increasingly important data point — the iPad accounted for 20% of Apple’s revenue in Q1, second only to the iPhone (53%) — is devilishly hard to predict, especially this quarter.

The new iPad had an impressive launch (3 million in 3 days) and an aggressive rollout (35 countries in two weeks), but racked up only a fortnight of sales before the quarter ended. Meanwhile, who knows how much iPad 2 purchases might have dwindled in advance of the new model or accelerated after the $100 price cut?

The real question may be whether Apple could make either model fast enough to meet demand.

The answers provided by the 50 analysts — professional and independent — whom we polled for this report were wildly divergent, both inside and outside their respective groups. For the first time since we’ve been tracking their numbers, the six best independent analysts came up with a consensus number that was actually lower than the professionals, who tend to be considerably more cautious. (See the yellow bar in the chart above.)

Among the 28 Wall Street analysts we heard from, iPad sales estimates ran from a low of 9 million from Morgan Stanley’s Katy Huberty to a high of 14.47 million from Topeka’s Brian White — an unusually large range of nearly 5.5 million units. The average estimate from this group was 11.87 million, which would represent an increase of 153% over Q2 2011’s 4.69 million.

The average estimate among our newly expanded list of independent analysts — 22 in all — was 12.2 million, a 181% increase from the same quarter last year. The low estimate in this group was 9.9 million from Alexis Cabot, an American expat living in Rome, and the high was 15.55 million, submitted by Gregg Thurman of AAPL Independent Analysts.

We like to keep an especially close eye on the six independent analysts with the best track record over the past six quarters. Their consensus: 10.95 million iPads, a year-over-year increase of 133%.

We’ll find out who was closest to the mark when Apple reports its earnings after the markets close next Tuesday, April 24.

Below the fold: The analysts’ individual estimates, with the pros in blue and the amateurs in green. Note the arrangement of colors. Usually the more bullish amateurs (in green) migrate to the top and the pros (blue) to the bottom; this time both groups are scattered all over the lot.