Communication Gateway App
This has three aspects, and would probably need to be a native app instead of a java app:
0) Wifi Access Point - like Joikuspot (or like Cradlepoint PHS and CTR devices), but for Android. Turns your Android phone into a wifi access point / personal hotspot. Also solves the tethering problem. (though, you might want to get an external battery for it, or an extended battery, but IMO it'd be worth it)
1) embedded Jabber server for SMS/MMS messaging - You could run a jabber client on your laptop/desktop and connect it to your phone. Your phone would then send your text/picture messages through this jabber connection.
2) embedded SIP server for calls - same as above, but for voice calls. This would let you receive calls from any SIP program (running on your laptop/desktop) or SIP device (some voip phones, some other phones, etc).
(#1 and #2 should be usable whether your phone is being used as an access point, or a wifi client; but if you're using it as a client, you'll need some way to tell your jabber/sip clients what your phone's IP address is, so that they can connect to it)
When you're on the move, you can use your phone directly for SMS/MMS/voice ... but when you get somewhere where you can pull out your laptop, or sit down at a desktop, you don't have to keep switching back and forth between computer and phone to continue a conversation. It all just seamlessly blends together, in terms of communication capability.