Huh? The DMG expressly calls out that [fire] powers set things on fire (in case it wasn't obvious in any event). Given that a lighting bolt spell creates a great bolt of lightning dealing lightning damage, I think it's pretty obvious that it could be used to send a signal.
I'm brand new to 4e, but I see absolutely no reason why it couldn't be used to send a signal, either. (Side note: my players tend to read the actual flavor printed on cards during combat. Not every attack by any means, but someone might say "what does that look like?" when a power is used, and the cards have seemed to be a natural starting point for them.)
As far as I know, too, the section on damaging objects says that there's only three types of damage that don't damage objects: poison, psychic, and necrotic. It seems like 4e relies on the use of keywords to dictate how powers interact with things during game play (as pemerton has noted a number of times). So, we have the following for the Lightning Bolt spell:
Compendium said:
Lightning Bolt
Brilliant strokes of blue-white lightning erupt from your outstretched hand.
Encounter Arcane, Evocation, Implement, Lightning
Standard Action Ranged 10
Target: One, two, or three creatures
Attack: Intelligence vs. Reflex
Hit: 2d6 + Intelligence modifier lightning damage.
Miss: Half damage.
If we look at the key words, we have the following:
Arcane: Drawing on magical energy that permeates the cosmos, the arcane power source can be used for a wide variety of effects, from fireballs to flight to invisibility.
Evocation: Evocation powers bring various magical effects into being, including explosions, rays of magical energy, and lingering environmental effects.
Implement: The implement keyword identifies a power that can be used through an implement, and the implement must be a type wielded by the power’s class. You can use implement powers without an implement, and wielding a nonmagical implement confers no benefit.
Lightning: Electrical energy.
So, taking all of that into account, we know that you draw on arcane magic and (usually) channel the magic through your implement (staff, wand, whatever) to bring electrical energy into being. The attack originates from you, and stretches up to 50 feet out (10 squares). As far as I can tell, there's no reason this couldn't be used as a signal in 4e, just like in any past edition; you're still creating a line of lightning, just as you did in prior editions.
Unless I missed something, of course. I'm open to other interpretations.