Putting together the advice given by egreg on questions Converting command to result, Storing environment arguments, What are the exact semantics of \detokenize? as well as this one, I managed to create what I wanted.
For the sake of completeness, I am providing the code. There are still some rough edges: task{0:15} will print 0 hrs 15 mins, the tables don't take up the whole \textwidth, etc. As a proof of concept, I am very satisfied.
report.tex
\documentclass{workreport}
\begin{document}
\begin{dayreport}{June 13, 2011}
\task{First task}{3}
\task{Second task}{1:12}
\task{Third task}{3:15}
\end{dayreport}
\begin{dayreport}{June 14, 2011}
\task{Fourth task}{1:15}
\task{Fifth task}{2}
\task{Sixth task}{1}
\end{dayreport}
\totaltime
\end{document}
Output

workreport.cls
% workreport.cls
\ProvidesClass{workreport}
\LoadClass[a4paper,12pt]{article}
% infix arithmetic
\usepackage{calc}
% setting section titles
\usepackage{titlesec}
\titleformat{\section}{\Large\scshape\raggedright}{}{0em}{}[]
% nicer looking table rules
\usepackage{booktabs}
% counters for time calculation
\newcounter{totaltime}
\setcounter{totaltime}{0}
\newcounter{dailytime}
\setcounter{dailytime}{0}
\newcounter{hours}
\setcounter{hours}{0}
\newcounter{minutes}
\setcounter{minutes}{0}
% required for xappto command
\usepackage{etoolbox}
\newcommand{\totaltimerows}{}
\newcommand{\totaltime}{
\section*{Total Work Hours}
\begin{tabular}{p{0.75\textwidth}r}
\toprule
Day & Work hours \\
\midrule
\totaltimerows
\bottomrule
\setcounter{minutes}{\thetotaltime-((\thetotaltime/60)*60)}
\setcounter{hours}{\thetotaltime/60}
\emph{Total work hours:} & \displaytime{\thehours:\theminutes}\\
\end{tabular}
}
% prints time and advances time counters, format is \tasktime{hh:mm}
\def\tasktime#1{\@tasktime#1::\@nil}
\def\@tasktime#1:#2:#3\@nil{%
% add hours time to counter
\setcounter{dailytime}{\thedailytime+#1*60}
% if there are no minutes, detokenize is empty so \relax=\relax
% output hour/hours
\if\relax\detokenize{#2}\relax
#1 hr\ifnum#1=\@ne\hphantom{s}\else s\fi
\else
% check if minutes are greater than 0
\ifnum#2>0
% add minutes to counter and output time
\setcounter{dailytime}{\thedailytime+#2}
#1 hr%
\ifnum#1=\@ne
% '\' is for spacing
\else s\fi \ #2 mins
% if the minutes are not greater than 0
\else
#1 hr\ifnum#1=\@ne\hphantom{s}\else s\fi
\fi
\fi}
% prints time but does not advance the time counters
\def\displaytime#1{\@displaytime#1::\@nil}
\def\@displaytime#1:#2:#3\@nil{%
\if\relax\detokenize{#2}\relax
#1 hr\ifnum#1=\@ne\hphantom{s}\else s\fi
\else
\ifnum#2>0
#1 hr%
\ifnum#1=\@ne
\else s\fi \ #2 mins
\else
#1 hr\ifnum#1=\@ne\hphantom{s}\else s\fi
\fi
\fi}
\newcommand{\task}[2]{#1 & \tasktime{#2}\\}
\newenvironment{dayreport}[1]{%
\section*{#1}
\appto\totaltimerows{#1 & }
\begin{tabular}{p{0.75\textwidth}r}
\toprule
Task & Approximate time \\
\midrule}{
\bottomrule
\setcounter{minutes}{\thedailytime-((\thedailytime/60)*60)}
\setcounter{hours}{\thedailytime/60}
\setcounter{totaltime}{\thetotaltime+\thedailytime}%
\setcounter{dailytime}{0}%
\emph{Total time:} & \displaytime{\thehours:\theminutes}\\
\end{tabular}
\xappto\totaltimerows{\displaytime{\thehours:\theminutes}\noexpand\\}
}