Built on jsMath, MathJax is a wonderful browser rendering engine for LaTeX, MathML and AsciiMath, which works fine in the native Android Browser, and also in Firefox for Android. Unfortunately, though, none of these technologies (except maybe AsciiMath) is fully supported at this time. For example, only certain LaTeX commands are supported. You can also read the documentation on MathML support and AsciiMath support.
If you're comfortable with the command line, you can also try
- TeX Live for Android, although it has been reported to not work with some devices.
- TeXPortal, another work in progress effort to port the complete TeX Live to Android.
So, there are alternatives and everything has its ups and downs. I just hope Google converts Android to a full Unix system, it'll make things much easier for everyone.