Online and multichannel retailers pulled out their big guns in an effort to entice shoppers to open their wallets this holiday shopping season.

Early sales and heavy discounts figured prominently in their strategies, leading some to wonder whether they’d do too much, too early, leading to a drop-off in sales as the season progressed.

We quickly learned that the strong start sparked by Thanksgiving Day sales didn’t apear to have a negative impact on Black Friday, and a strong Black Friday didn’t stop consumers from spending on Cyber Monday.

But how are things going now?

According to comScore, shoppers are still going strong. In fact, as of yesterday, there have been six individual days on which U.S. consumers racked up over a billion dollars in online sales. The total haul for the entire season thus far: just under $25bn.

And comScore doesn’t think we’ve seen the last of the spending. All told, the four days with the heaviest online spending in U.S. history have occurred this year, and it’s predicting that today’s Green Monday sales could be a big one.

If today’s sales surpass $1.028bn, 2011 would account for the top five heaviest online shopping days in U.S. history.

That would obviously be read as a positive economic sign generally, but it probably says more about the state of online retail. Clearly, some sort of tipping point has been reached.

Needless to say, more consumers than ever before are comfortable doing their holiday shopping from the comfort of their home or office. But perhaps more importantly, online and multichannel retailers are clearly in a position to serve them well, offering them products (and deals) that are appealing, and getting them to click ‘Complete Purchase‘.

It has been a long time coming but unless there’s a disastrous end to the holiday shopping season, 2011 may go down as the breakthrough year for online retail.