Phi, Shomen uchi is a very, very specific strike, as is all the other ones (yokomen uchi, jodan zuki, etc). They're for training purposes and are simulated attacks. In no way are these karate's seiken zuki or shuto uke. If you got a bottle in your hand, or a hammerfist, that's no longer a kihon waza's initiating strike, but a full on attack that has to be dealt with differently.
Graham, I agree that the shomen uchi is a sword cut, but it's with the tegatana, and you know how effective those are dispatching folks to the other side of the river.
I think the main problem you guys are having with what I'm saying is that you think a shomen uchi is an attack. No, it's not, but that's just definitions. The important thing to note is that you don't use something like this as an actual attack, in a battle, because as an Aikidoka, if you extend yourself out of your "box" where you are strongest in (centre power area), then you become uke instead of nage (shite). You can easily see the extension of body in yokomen uchi, but the same holds for other forms of simulated strikes. Now, you can certainly use things similar to the shomen uchi by utilizing your tegatana, but the strike will be modified to work effectively without putting yourself at a disadvantage via opening yourself to attacks or kuzushi. This commonly occurs during a technique, in close range, rather than as the initiating strike.
The thing about strikes like shomen uchi, yokomen uchi, etc., is that they are done on purpose to allow for techniques to be used on them. They simulate not the speed nor power, but the line of attack/flow that other arts would use. This allows for us to train in recieving and countering the strikes. But as uke, you need to cooperate with nage in completing the technique, so one way to do this without being totally non-resistant is to offer a small opening for the technique to take hold, either as an extension of the body, or a small imperfection in the stance, allowing for nage to apply blocks, evasions, atemi, or kuzushi.
But as I've said before, I'm no Shihan, so everything I said can be disregarded by others of higher credibility at will. I've posted it just as a food for thought or a launching pad for discussion. Rather than talking about the usual lacking-in-experience comment about how Aikido is useless for self-defense (the Kidotai would like to disagree with you, as are your friendly local senshusei grads), let's discussion about something that interests us and educates others (or at least show them how much they don't know about Aikido