Is it possible to have placeholders in strings.xml for runtime values?

Is it possible to have placeholders in string values in string.xml that can be assigned values at run time?

Example:

some string PLACEHOLDER1 some more string

Formatting and Styling

Yes, see the following from String Resources: Formatting and Styling

If you need to format your strings using String.format(String, Object...), then you can do so by putting your format arguments in the string resource. For example, with the following resource:
<string name="welcome_messages">Hello, %1$s! You have %2$d new messages.</string>
In this example, the format string has two arguments: %1$s is a string and %2$d is a decimal number. You can format the string with arguments from your application like this:
Resources res = getResources();
String text = String.format(res.getString(R.string.welcome_messages), username, mailCount);

Basic Usage

Note that getString has an overload that uses the string as a format string:

String text = res.getString(R.string.welcome_messages, username, mailCount);

Plurals

If you need to handle plurals, use this:

<plurals name="welcome_messages">
    <item quantity="one">Hello, %1$s! You have a new message.</item>
    <item quantity="other">Hello, %1$s! You have %2$d new messages.</item>
</plurals>

The first mailCount param is used to decide which format to use (single or plural), the other params are your substitutions:

Resources res = getResources();
String text = res.getQuantityString(R.plurals.welcome_messages, mailCount, username, mailCount);

See String Resources: Plurals for more details.

Supplemental Answer

When I first saw %1$s and %2$d in the accepted answer, it made no sense. Here is a little more explanation.

They are called format specifiers. In the xml string they are in the form of

%[parameter_index$][format_type]

%: The percent sign marks the beginning of the format specifier.
parameter index: This is a number followed by a dollar sign. If you had three parameters that you wanted to insert into the string, then they would be called 1$, 2$, and 3$. The order you place them in the resource string doesn't matter, only the order that you supply the parameters.
format type: There are a lot of ways that you can format things (see the documentation). Here are some common ones:
s string
d decimal integer
f floating point number

Example

We will create the following formatted string where the gray parts are inserted programmatically.

My sister Mary is 12 years old.

string.xml

<string name="my_xml_string">My sister %1$s is %2$d years old.</string>

MyActivity.java

String myString = "Mary";
int myInt = 12;
String formatted = getString(R.string.my_xml_string, myString, myInt);

Notes

I could use getString because I was in an Activity. You can use context.getResources().getString(...) if it is not available.
String.format() will also format a String.
The 1$ and 2$ terms don't need to be used in that order. That is, 2$ can come before 1$. This is useful when internationalizing an app for languages that use a different word order.
You can use a format specifier like %1$s multiple times in the xml if you want to repeat it.
Use %% to get the actual % character.
For more details read the following helpful tutorial: Android SDK Quick Tip: Formatting Resource Strings