Use Google Calendar. Make your calendar public. "Tweet" by adding an event; if you simply text it to GCal, it's put in the calendar at the point you sent it in.
Have your friends do the same thing and share their GCals with you. Show all their calendars in your default view. You'll see your "tweets" and theirs in an ordered time stream. They'll also be auto color-coded, although you'll soon run out of colors.
- Massive, distributed infrastructure.
- SMS integration (upstream).
- Many clients exist for desktop and mobile.
- Can export to other services and perhaps import from other services.
- Calendars (streams) will not be easy to scale.
- No practical SMS integration downstream at present.
- This is really, really not what GCal is designed to be used for.