For the first, beamer uses templates (itemize item, itemize subitem, itemize subsubitem) to typeset the item symbol. See the section "Itemizations, Enumerations, and Descriptions" (11.1 in my version) in the beamer manual. If you read the source files beamerinnerthemedefault.sty and beamerbaseauxtemplates.sty you can see the syntax for declaring other choices for this template. So to define ticks (by which I understand checkmarks) and cross item icons you can do something like this:
\usepackage{pifont}
\defbeamertemplate{itemize item}{cross}{\scriptsize\raise1.25pt\hbox{\donotcoloroutermaths\ding{54}}}
\defbeamertemplate{itemize subitem}{cross}{\tiny\raise1.25pt\hbox{\donotcoloroutermaths\ding{54}}}
\defbeamertemplate{itemize subsubitem}{cross}{\tiny\raise1.25pt\hbox{\donotcoloroutermaths\ding{54}}}
\defbeamertemplate{itemize item}{tick}{\scriptsize\raise1.25pt\hbox{\donotcoloroutermaths\ding{52}}}
\defbeamertemplate{itemize subitem}{tick}{\tiny\raise1.25pt\hbox{\donotcoloroutermaths\ding{52}}}
\defbeamertemplate{itemize subsubitem}{tick}{\tiny\raise1.25pt\hbox{\donotcoloroutermaths\ding{52}}}
Then if you wanted to make those icons the default you would just do:
\setbeamertemplate{itemize item}[cross] % and so on.
But to use your custom icons to highlight the current item you can do this:
\documentclass{beamer}
\useinnertheme{rectangles}% or whatever inner theme you want
% template defs from above
\makeatletter
\newenvironment{crossenv}{%
\only{%
\beamer@computepref\@itemdepth% sets \beameritemnestingprefix
\setbeamertemplate{itemize \beameritemnestingprefix item}[cross]
}% overlay/action specification gets added here by beamer
}{%
}
\newenvironment{tickenv}{%
\only{%
\beamer@computepref\@itemdepth% sets \beameritemnestingprefix
\setbeamertemplate{itemize \beameritemnestingprefix item}[tick]
}% overlay/action specification gets added here by beamer
}{%
}
\makeatother
\begin{document}
\begin{frame}{Single list}
\begin{itemize}
\item<1- | tick@+-> foo
\item<1- | cross@+-> bar
\item<1- | tick@+-> foo, again
\end{itemize}
\end{frame}
\begin{frame}{Nested lists}
\begin{itemize}[<1-| cross@+>]
\item foo
\item bar
\begin{itemize}
\item bar one
\item bar none
\begin{itemize}
\item bar who?
\item bar none
\end{itemize}
\end{itemize}
\item baz
\end{itemize}
\end{frame}
\end{document}
Because the crossenv enviroment surrounds each selected item, the change in template is local. After the crossenv environment the template returns to whatever it was before. This means you don't need to know what the symbol is by default. So this will work with any inner theme you are using.
Final Edits: I think I finally got what you wanted. You have to only change the template at the current itemize level, which requires a bit of h@ckery. But diving into beamerbaselocalstructure.sty revealed a solution.
Final Final Edits: Now with ticks and crosses both.
It's a sync problem; if you add \thebeamerpauses in some places (to get the value of the beamerpauses counter), you can see what's going on:
\documentclass{beamer}
\usepackage[utf8]{inputenc}
\usepackage{default}
\begin{document}
\begin{frame}
\begin{itemize}[<+->]
\item A\thebeamerpauses
\item B\thebeamerpauses
\item C\thebeamerpauses
\end{itemize}\thebeamerpauses
\pause
Some text.\thebeamerpauses
\end{frame}
\end{document}
At the end of the itemize environment the counter has a value of four, and \pause steps it to 5, so the text will appear on the fifth slide.
To obtained the expected result you can use the optional argument of \pause and the value of the beamerpauses
counter:
\documentclass{beamer}
\usepackage[utf8]{inputenc}
\usepackage{default}
\begin{document}
\begin{frame}
\begin{itemize}[<+->]
\item A
\item B
\item C
\end{itemize}
\pause[\thebeamerpauses]
Some text.
\end{frame}
\end{document}
Another option is to use \onslide<+-> instead of \pause:
\documentclass{beamer}
\usepackage[utf8]{inputenc}
\usepackage{default}
\begin{document}
\begin{frame}
\begin{itemize}[<+->]
\item A
\item B
\item C
\end{itemize}
\onslide<+->{Some text.}
\end{frame}
\end{document}
Best Answer
The following MWE provides
\hidecontent[<transparency>]{<stuff>}that hides<stuff>with a white transparent box. Optional transparency is set as<transparency>(provided by Heiko'stransparentpackage); default is0.25or 25%:beamerdoesn't adequately handle hiding of graphics, hence the use of\hidecontent. This works well with rectangular content. Other content (textual, for example), is best dealt with bybeameritself.