Mobile payments at the point of sale will top $1 billion this year, according to a new forecast from eMarketer, nearly doubling last year’s $54 million. And the market will “expand rapidly” over the next few years, the firm predicts, with Americans ringing up $58 billion in proximity mobile payments by 2017.

But as All Things D points out — and as eMarketer acknowledges in its press release — the latest forecast is drastically less optimistic than one the same company issued less than a year ago, which predicted U.S. consumers would conduct $2.12 billion in proximity mobile payments in 2013. “Delays and adoption issues facing numerous mobile wallet initiatives” are to blame for the slow uptake, eMarketer claimed, “as well as a congested landscape of competing technologies….”

There’s no question that the overall market for mobile payments is being shackled by competing technologies and systems that don’t play well together and are each backed by big corporations with much at stake. (I first wrote about that in depth in this column from March 2012.) And while some progress is being made in the space — Apple’s Passbook and Starbucks Rewards prove that loyalty programs will play a huge role as the space evolves, for instance — plenty of other major challenges remain. Which is why I won’t be surprised if eMarketer is forced to slash its forecast again next year.