I'd use 9\textsuperscript{00}--10\textsuperscript{20} rather than $9^{00}-10^{20}$ because:
- It's not math.
- It uses the math font rather than the text font.
$-$ is a minus sign with its appropriate spacing while -- is the correct en dash (“9 o'clock to 10.20” not “9 raised to the 0-th power minus 10 raised by the 20-th power”). \end{nitpick} Compare:

White Space
I still believe this is a PDF viewer rendering issue.
When viewing with SumatraPDF at 6400 % (!) I can see the white-ish (they are grey) lines (here at the crossing of the hhlines):

At around 1000 % you can still see them:

These are (anti-)aliasing artefacts.
Looking closely at the 6400 % image you can see that there's a lot going on:

With the Foxit Reader some of these white spaces are not visible at different zoom factors.
Adobe Reader doesn't show them at all (example at 6400 %):

Now your \\[-1pt] trick does help you in the way, that colored areas do overlap (by 1pt nonetheless) instead of just touching themselves. But don't use it; they're really just touching.
I can also resolve this non-existing issue by (virtually) printing to PDF.
You may have noticed that these thin white spaces are the same size at every zoom factor which is another indicator that this is a rendering issue.
Bigger \hhline.
Apart from setting a tabular-global \arrayrulewidth there, as far as I know, are two solutions (meaning one bad solution and one work-around):
The following may not apply to your specific table …
Instead of using a header which describes the columns in a very duh! way (Really? Mon, Tue, Wed, … do stand for (week)days? Oh, and 900–1020 is a time (span)?), I'd use a combination of booktabs and no colors.†
Further effort has to be put in the goal of same-width columns (tabularx' X specification for example). In my own timetable I even have dropped the weekdays headings, because I am (just barely) able to count to five. ;)
† See Herbert's comment: “from typographers view it makes no sense to have lines and colored cells”, and Alan Munn's answer.
Code
\documentclass{article}
\usepackage{multirow}
\usepackage{tgheros}
\usepackage{booktabs}
\renewcommand*\familydefault{\sfdefault}
\begin{document}
\centering
\begin{tabular}{r*5c}
\toprule
& Mon & Tue & Wed & Thu & Fri \\ \midrule
9\textsuperscript{00}--10\textsuperscript{20} & \multirow{2}{*}{English} & Mathematics & Deutsch & \TeX & --- \\
10\textsuperscript{30}--11\textsuperscript{50} & & Physics & Italiano & \LaTeX & --- \\ \bottomrule
\end{tabular}
\end{document}
Output

tabularis possible through\setlength{\arrayrulewidth}{<len>}where you pick<len>. – Werner Oct 27 '12 at 20:29\hhlinethickness. – Ptech Oct 27 '12 at 21:04