It'll probably be the same thing with Napster… at the end of this all, they'll make a manga file-sharing server that will be a legitimate business: 12.95 a month for unlimited manga downloads.
One problem with things like 'New Napster' is that they tend to ignore what made the origjnal work so well: the decentralized nature means it scales up VERY easily and cheaply, and the offerings will reflect the tastes of the actual customers. And they can't have decentralized without giving up some control.
Can you imagine if everyone hit a single site offering the new issue at the same time? They'd need ten times the infrastructure, one day a week.
On the subject though, an ad-supported online reading system would probably work well (maybe using a custom applet to make "scraping" more difficult and to manage the distribution network to shift the load, maybe embedding advertisements in blank portions of pages or in margins)
The math would be interesting. If a typical user reads even five out of the 20 or so Jump series a week, that's at least 100 impressions per user per week, plus any extras they want to read (or back chapters, or re-reading).
As of 2005, per http://www.comipress.com/article/2007/05/06/1923, WSJ has a circulation in the range of 3 million copies.
Also add foreigners who wouldn't buy Jump directly at "210 yen plus +$10 postage", and recapturing people who'd buy used copies or skip issues when they can't get or afford them new.
We're talking tens of millions of pageviews per week, if even one in thirty WSJ buyers goes to the online option. While that's not a lot compared to the absolute biggest sites on the web (MySpace boasts just over 10 BILLION pageviews a week), remember that this IS a niche product (I could actually see it being tailor-made for keyword advertising)
A pay-to-read system would almost certainly have issues with international purchases, and it will be hard to sell, because for under $9 per month (5 issues @ 210 yen each = ~$8.60), people in Japan can get a physical copy of Jump each week that they can read in the bathroom, burn the Naruto chapter, and tape the colourspreads to their walls. You couldn't price an online service low enough to be a real winner against that, and still cover expenses directly.