Being a long term Java / Mac developer I am only too well aware that support for Java in all it's versions is often lagging behind Windows. For me I am worried that there won't be support for Java 7 JDK in Yosemite.
My development toolkit includes Java 7, mySQL, Eclipse & WindowBuilder & JasperReports, Apache Tomcat. Some of these components are not Java 8 compliant as yet - and even if they were I still prefer to use the same version of Java as my customers. I read some article that the Java 7 installer complains about the OSX version being out of date if you try and install it on Yosemite.
I think that this has to do with the version number of the public beta not being a standard one that the installer is not able to determine. It requires a minimum version of 10.7.3. To fix this in public beta, just do the following
Go to System->Library->CoreServices inside your OS X hard drive and open the file "SystemVersion.plist" with TextEdit. Change the system version listed under "ProductVersion" to whatever you desire.
You might not be able to write to the file because it's system-protected (in Lion, TextEdit will let you unlock system files). To work around this problem, "Save As" this file to the desktop, or some other random place where you can find it. While saving it, make sure you uncheck the option to convert it into a .txt file.