\newcommand for tabular entries using keyval

Here is how it works:

\documentclass{article}

\usepackage{keyval}    

\makeatletter
\define@key{tabularEntryKeys}{a}[\dots]{\gdef\A{#1}}
\define@key{tabularEntryKeys}{b}[\dots]{\gdef\B{#1}}
\makeatother

\newcommand\tabularEntry[1]{\setkeys{tabularEntryKeys}{#1} \A & \B}

\begin{document}

\begin{tabular}{c|c}\hline
    \tabularEntry{a=1,b=2}
\end{tabular}

\end{document}

Here is one more approach. It does neither global definitions nor use of e-TeX extensions and \edef.

\documentclass{article}

\usepackage{keyval}
\def\TAB{&}

\makeatletter
\define@key{tabularEntryKeys}{a}[\dots]{\def\tek@rowA{#1}}
\define@key{tabularEntryKeys}{b}[\dots]{\def\tek@rowB{#1}}
% initialize
\def\tek@rowA{}
\def\tek@rowB{}

\newcommand{\tabularEntry}[1]{\setkeys{tabularEntryKeys}{#1}%
                              \tek@rowA \expandafter\TAB\tek@rowB }
\makeatother

\begin{document}

\begin{tabular}{c|c}
\hline
  \tabularEntry{a=1,b=2} \\
  \tabularEntry{a=1} \\
  \tabularEntry{b=2} \\
  \tabularEntry{a,b} \\
  \tabularEntry{a} \\
  \tabularEntry{b} \\
  \tabularEntry{} \\
\end{tabular}
% \begin{tabular}{c|c}\hline
%   \tabularEntry{a=1,b=2}\\
%   \tabularEntry{a=3,b=7}\\
%   \tabularEntry{a=10,b=20}
% \end{tabular}

\end{document}

In case of more cells in one row, one needs a variant to avoid many many \expandafter's: (notice the extra spaces added before #1 and in the initial defaults values except for \tek@rowA and \tekrowD)

\usepackage{keyval}
\def\TAB{&}
\makeatletter
\define@key{tabularEntryKeys}{a}[\dots]{\def\tek@rowA{#1}}
\define@key{tabularEntryKeys}{b}[\dots]{\def\tek@rowB{ #1}}
\define@key{tabularEntryKeys}{c}[\dots]{\def\tek@rowC{ #1}}
\define@key{tabularEntryKeys}{d}[\dots]{\def\tek@rowD{#1}}
% initialize
\def\tek@rowA{}
\def\tek@rowB{ }
\def\tek@rowC{ }
\def\tek@rowD{}

\newcommand{\tabularEntry}[1]{\setkeys{tabularEntryKeys}{#1}%
     \tek@rowA \expandafter\TAB
     \romannumeral0\expandafter\tek@rowB\expandafter\TAB
     \romannumeral0\expandafter\tek@rowC\expandafter\TAB
     \tek@rowD 
}
\makeatother

Your \setkeys command can't straddle two cells. On the other hand globally defining \A and \B doesn't seem a good idea.

\documentclass{article}

\usepackage{keyval,etoolbox}    

\makeatletter
\define@key{tabularEntryKeys}{a}[\dots]{\def\tek@rowA{\unexpanded{#1}}}
\define@key{tabularEntryKeys}{b}[\dots]{\def\tek@rowB{\unexpanded{#1}}}
% initialize
\def\tek@rowA{}
\def\tek@rowB{}
\newcommand\tabularEntry[1]{%
  \setkeys{tabularEntryKeys}{#1}%
  \edef\tek@row{\tek@rowA & \tek@rowB}\tek@row
}
\makeatother

\begin{document}

\begin{tabular}{c|c}
\hline
  \tabularEntry{a=1,b=2} \\
  \tabularEntry{a=1} \\
  \tabularEntry{b=2} \\
  \tabularEntry{a,b} \\
  \tabularEntry{a} \\
  \tabularEntry{b} \\
  \tabularEntry{} \\
\end{tabular}

\end{document}

Note that the optional argument in \define@key is the value assigned to the key when the key is specified without =<value> following, not an initial value.

If you want the values to repeat if a key is not specified, then say

\define@key{tabularEntryKeys}{a}[\dots]{\gdef\tek@rowA{\unexpanded{#1}}}
\define@key{tabularEntryKeys}{b}[\dots]{\gdef\tek@rowB{\unexpanded{#1}}}

but don't use “easy” macro names such as \A or \B.