\documentclass{article}
\begin{document}
\begin{enumerate}
\item first;
\item second;
\end{enumerate}
This is a paragraph.
\begin{enumerate}
\setcounter{enumi}{2}
\item third;
\item and so on...
\end{enumerate}
\end{document}
edit: as pointed out by Dervin Thunk, I hardcoded 2 here.
so, here's a solution that seems to work:
\documentclass{article}
\newcounter{tempcounter}
\begin{document}
\begin{enumerate}
\item first;
\item second;
\setcounter{tempcounter}{\value{enumi}}
\end{enumerate}
This is a paragraph.
\begin{enumerate}
\setcounter{enumi}{\value{tempcounter}}
\item third;
\item and so on...
\end{enumerate}
\end{document}