Is typing %^ into cmd.exe a Windows easter egg?

No.

Microsoft formally stopped including Easter eggs in its programs as part of its Trustworthy Computing Initiative in 2002.
http://en.wikipedia.org/wiki/Easter_eggs_in_Microsoft_products

Larry Osterman at Microsoft noted in October 2005 that adding an Easter Egg is grounds for termination.

Nowadays, adding an easter egg to a Microsoft OS is immediate grounds for termination, so it's highly unlikely you'll ever see another.

http://blogs.msdn.com/b/larryosterman/archive/2005/10/20/483110.aspx

The command prompt is looking for a continuation (More?) of the command, since it ended with the escape character ^.

The ^ escape character can be used to make long commands more readable by splitting them into multiple lines and escaping the Carriage Return + Line Feed (CR/LF) at the end of a line
http://ss64.com/nt/syntax-esc.html

CMD is line based. It only reads and executes one line at a time. When you are typing and you haven't finished a line it prompts with More?.

Your specific thing is there is no line ending so it's waiting to see what comes after the % sign.

It's easier to see using brackets

Try dir

then

(dir
echo %time%
(type c:\windows\win.ini
)
)

Only when the line is complete (matching brackets) is it read and executed.

Here's a list of punctuation.

&    separates commands on a line.

&&    executes this command only if previous command's errorlevel is 0.

||    (not used above) executes this command only if previous command's errorlevel is NOT 0

>    output to a file

>>    append output to a file

<    input from a file

|    output of one command into the input of another command

^    escapes any of the above, including itself, if needed to be passed to a program

"    parameters with spaces must be enclosed in quotes

+ used with copy to concatenate files. E.G. copy file1+file2 newfile

, used with copy to indicate missing parameters. This updates the files modified date. E.G. copy /b file1,,

%variablename% a inbuilt or user set environmental variable

!variablename! a user set environmental variable expanded at execution time, turned with SelLocal EnableDelayedExpansion command

%<number> (%1) the nth command line parameter passed to a batch file. %0 is the batch file's name.

%* (%*) the entire command line.

%<a letter> or %%<a letter> (%A or %%A) the variable in a for loop. Single % sign at command prompt and double % sign in a batch file.

\\ (\\servername\sharename\folder\file.ext) access files and folders via UNC naming.

: (win.ini:streamname) accesses an alternative steam. Also separates drive from rest of path.

. (win.ini) the LAST dot in a file path separates the name from extension

. (dir .\*.txt) the current directory

.. (cd ..) the parent directory


\\?\ (\\?\c:\windows\win.ini) When a file path is prefixed with \\?\ filename checks are turned off. 

< > : " / \ | Reserved characters. May not be used in filenames.



Reserved names. These refer to devices eg, 

copy con <filename> 

which copies a file to the console window.

CON, PRN, AUX, NUL, COM1, COM2, COM3, COM4, 

COM5, COM6, COM7, COM8, COM9, LPT1, LPT2, 

LPT3, LPT4, LPT5, LPT6, LPT7, LPT8, and LPT9



Maximum path length              260 characters
Maximum path length (\\?\)      32,767 characters (approx - some rare characters use 2 characters of storage)
Maximum filename length        255 characters


.
--

Simple really, From all the other answers and comments (and some inputs of my own) this is what I gathered:

C:\windows\system32>net ^
More? user

User accounts for \\INFINITEPC

-------------------------------------------------------------------------------
Administrator            Guest                    Rahul
The command completed successfully.