I'm going to be curious to see what it looks like, and I wouldn't be suprised if they took a look at Fallout 3 for some aesthetic inspiration (any success of Interplay's MMO, let's face it - is going to be built more off of Fallout 3's popularity than a familiarity with Fallout 1 or 2.) But I also wouldn't assume much purposeful resemblance to Fallout 3.
I mean, I wouldn't be suprised if it played out with a 3rd-person camera (which seems to be the standard for an MMO) as opposed to an isometric view; and if it didn't end up looking a bit like Fallout 3 in many ways (as they're both going to be highly detailed renderings of a barren Wasteland - a dead tree, some rubble, and lots of sand are going to look a least a little similar regardless of the engine used.)
I'd also assume that it's going to use the SPECIAL system (might be interesting - the MMO might be more faithful to it's original interpretation than the current sequel
.) And it's probably more likely that combat will be handled kind of in the same manner (targeting enemies with the reticule - regardless of whether it's just for determining your target or factoring into accuracy; character vs player skill and all that - as opposed to a turn-based sort of interface.)
Chances are, the end result is going to draw more comparisons to Fallout 3 than Fallout 1 - just by virtue of the way games are made these days. But I'd also be quite suprised if it played exactly like Fallout 3 but with more people, as well. I wouldn't imagine that's what they're going for. Bethesda took Fallout 3 and made it into the game they wanted, I would assume the same sort of thing is going to take place with Interplay.