exactly...
@projectle - this thread is not iphone specific. the feature worked with any bluetooth phone supported by isync / addressbook, years before iphones existed. i used the feature from early g4 days (sony t610, lotsa nokias) and it rocked. until leopard killed it.
i for one don't care if this doesn't work on iPhone, which isn't even released in my country yet. the iphone release (and their not making this feature work with iphone) is exactly why the feature had to die - it wouldn't look good for apple to ship a product which lacked a feature that every bluetooth nokia / sony / ... has
so they punished all macos users by yanking the excellent functionality of being able to send/receive SMS via your phone (not your iphone, your every-other-brand phone) straight from addressbook. this to avoid offending the much smaller number of iphone users who might have missed the feature. (i love my ipod touch, but i wouldn't want an os feature killed just so i didn't feel like i missed out.)
apple killed this intentionally; i doubt it's coming back in 10.5.2 unless something radical happens in iphone land. discussion of this topic is censored in apple support forums (my thread was killed twice because "feature requests don't belong in a support forum" - WTF - you killed the feature, you bastards!)
i will be delighted to be proved wrong here - especially if we get the feature back! this one missing feature is worth ten of leopard's "three hundred". (yeah, i'm riled alright.)
there are open-source apps which implement AB plugins like this (eg adium, gizmo), and likewise toolkits (gnokii etc) which handle delivering/receiving the SMS
polling for receipt might be harder, mind, but not impossible
i'd love to be involved in lashing this together, but i'm no mac developer, more familiar with stringing linux tools together. would be dead keen to hear from others interested in making this happen. will keep an eye on this!