Not really. Let's look at a few long in-season hiatuses. All these are first-season shows except for Lost, which I just threw on because its "pod" scheduling that year was particularly infamous.
Fall = Change from season premiere to fall finale
Hiatus = Change from fall finale to spring premiere
Spring = Change from spring premiere to season finale
The best analogs here are Flashforward/V, both new shows that dropped a lot more while they were on the air than they did over the hiatus. Flashforward was canned and V was barely renewed. I'd expect The Event to follow their model. Maybe it returns somewhat optimistically (1.9 fall finale, so maybe a 1.8?), but as those two shows prove, weathering the hiatus storm does not remotely mean you're done declining. And while I tried to keep this to long hiatuses, many other shows with shorter hiatuses, like Chuck and Heroes in 2008-09 and Fringe this year, have gone a similar route. Those three actually rose across the hiatus but then took big drops after.
Can a hiatus really "kill" a show? I think you can make a real argument that happened with Jericho, which was steady right around a 3.0 for most of its fall run and then pretty steady in the low 2's immediately after its return. But that seems the exception and not the rule. The typically heavy promotion gives most shows a chance to return from hiatus pretty close to where they left. (And anyway, Jericho was famously axed and then resurrected, so it didn't really kill it, but it was obviously another close call.)
I'd say the hiatus itself probably won't "kill" The Event. Much like with last year's Flashforward, the only ultimately axed show of the above, The Event was struggling by the end of the fall. The only way you could really blame the hiatus is if it opens with something like a 1.2 or 1.3 tonight and then maintains that throughout. I don't expect that. The trend with this show (as with Flashforward) has been the more it airs, the more it drops.