how can i set the rounded corner look for different components in silvelright 2, i.e text box, button, drop down... ???

Just about every SL control allows you to customize the default look & feel via templates.  There are some videos on this website in the 'Learn' section that demonstrate this technique (on a button control I think).  It will help to do this type of customization, if you also are using Expression Blend (in my opinion) because much of the templating work is design work.

Thanks,
John
myOSity Project Website, Blog

could you please refer me to exact video, i have seen some, but could not find what i am looking for.

Here http://silverlight.net/learn/videocat.aspx?cat=2#HDI2Controls

Good luck

i already visited "controls" videos, i could not find the video which shows how to round the corners of textbox or butoon, etc. although i found one video in which he showed to create a rectangle with rounded cornerns and create a textbox inside it wityh no border, which is not a nice a way to achieve this, is there any better way. ?????

First you have to crerate TextBox:

FontFamily="Comic Sans MS" VerticalAlignment="Top"

Style="{StaticResource TextBoxStyle}" Foreground="#FFFFFFFF" BorderBrush="{x:Null}" Margin="0,5,0,5" HorizontalContentAlignment="Right" Grid.ColumnSpan="1" Grid.Column="1" HorizontalAlignment="Stretch" Width="Auto"/>

And then you need to create TextBoxStyle that will target TextBox inside the Page.XAML or App.XAML.