As promised, Google released the software development kit (SDK) for its new mobile operating system Android on Monday, and with the code in the wild for less than 24 hours, impressions are starting to roll in.

Forget about the $10 million developer contest. What’s more interesting is Google’s wranglings with Java – the language and open-source process – and early looks at potential Android user interfaces, applications and hardware.

On the Java front, Google has been planning to build a “custom” virtual machine (VM) for running Android apps. With the SDK’s release, Google has provided more details on that VM, which it is calling “Dalvik.”

A VM works by acting as a software-based “run-time” for turning raw code into actual applications. While Android developers will use Java tools – such as the open-source Eclipse IDE -- and the Java language to write Android apps, they will run not on standard phone Java Micro Edition (ME) virtual machines but on Dalvik. It is unlikely that apps written for one VM would run on the other.

For Google, that approach lets it tap into a huge group of existing Java developers while ensuring that Android phone apps are unique compared with apps written using Java ME.

To accompany Dalvik, Google is writing Java code libraries to help phone developers, including support for OpenGL 3D graphics and Bluetooth and USB interfaces. It is also providing hooks into several Google Web APIs, including Google Maps for displaying maps and XMPP for device-to-device communications.

The other open question regarding Android is the open-source license. While Google has licensed the technology using an Apache-style license – which allows developers wide range to take the code, change it and not “check” those changes back into the core – the search giant said it has required the members of its Open Handset Alliance to agree not to fragment the core Android code, as a condition of membership. OHA members include carriers such as Sprint and T-Mobile and manufacturers including HTC and Motorola.

Such an agreement would help limit Android from fragmenting but would not stop any developer or vendor that has not signed on as an OHA member to potentially take Android in whatever direction he or she chose.

Google focused strongly on building a strong technical platform, rolling out support for gesture-based touch screens, 3D acceleration and a variety of hardware form factors as part of Android’s core capabilities. On the Web side -- a major point of emphasis for Android – the platform is using the open-source WebKit browser engine, which among other deployments is the underlying code for Apple’s Safari browser.