With Minecraft version 1.3, which is currently scheduled to release on August 1, 2012, Mojang will make the game's single-player mode a "shell" on top of multiplayer. This step is being taken in advance of the upcoming modding API, and it also means that future bug fixes won't be specific to either mode.
The developer notes that online play itself ought to be a "smoother and more stable experience" as a result of 1.3. Single-player worlds can be shared on your local network and you'll also be able to use new commands similar to those available in multiplayer. The downside? Higher resource requirements.
Optimization is being worked on but might not make it until the next major update. Other notable features of 1.3 include the ability to write in books, tripwire, new terrain features, and the option to start Minecraft with a bonus chest for a quicker start. Here's the full list of what is included.