It's not nice that my mobile operator charges me 50 cents for any SMS that goes abroad, while using my internet voip account it costs me a couple of cents.
Moreover, some voip operators allow to send an SMS by a GET request with a URL that contains the account's credentials, the senders phone number and the message itself. See the section Using the VoipDiscount website:
http://www.voipdiscount.com/en/sms_instructions.html

So it is basically very easy to create a local HTML form that would allow to send an SMS from a phone through their service. But such a solution does not satisfy me because it's very inconvenient to type a senders phone number every time, while I already have him in my contacts. It would be perfect if I could simply reply on someone's SMS and it'd automatically go via the voip account.

If someone guides me I could try to develop such a thing myself since I am a software developer. I've had N900 for only few weeks and only yesterday installed the SDK, so I guess it would not go fast.
But from the other hand, I am quite motivated, because I don't like to be ripped off

There are already two applications supporting VoipDiscount and friends: WebTexter and Web2SMS.

Sending via the closed-source conversations app is not possible with both, but they insert the messages sent there.

Oh, thanks a lot!
I wouldn't want to discover America a second time.

Is it possible to use the stadard messaging interface, just route all the outgoing SMS via the web?

Nobody found out how or it was too much work (everyone's lazy ). There is a way to write rtcom-account plugins but that's too c'ish and gtk'ish for me ...