You can do this, you just need to circumvent the launcher.
In %appdata%\.minecraft\bin
(or ~/.minecraft/bin
on unixy systems), there is a minecraft.jar file. This is the actual game - the launcher runs this.
Invoke it like so:
java -Xms512m -Xmx1g -Djava.library.path=natives/ -cp "minecraft.jar;lwjgl.jar;lwjgl_util.jar" net.minecraft.client.Minecraft <username> <sessionID>
Set the working directory to .minecraft/bin
.
To get the session ID, POST (request this page):
https://login.minecraft.net?user=<username>&password=<password>&version=13
You'll get a response like this:
1343825972000:deprecated:SirCmpwn:7ae9007b9909de05ea58e94199a33b30c310c69c:dba0c48e1c584963b9e93a038a66bb98
The fourth field is the session ID. More details here. Read those details, this answer is outdated
Here's an example of logging in to minecraft.net in C#.