Create a PGraphics and draw the string on it.
But I doubt you will get a much better quality after rotation.

Confirmed.  I tried it, and the font quality really drops by drawing it to a PImage, since it doesn't seem to use the native fonts. 
I suppose there's no way to "trick" Processing/Java into letting me selectively use native fonts for some parts of an applet and bitmapped (.vlw) fonts for other parts of an applet?  Or does it do that automatically, and I just can't tell, because of the distorted output I'm getting on my Mac?

1. void setup()
   
2. {
3.   size(800, 200);
4. 
   
5.   PFont f = createFont("Times New Roman", 48);
6.   background(255);
7.    
8.   stroke(#FF0000);
9.   line(0, 100, width, 100); // Just testing transparency...
   
10. 
    
11.   textFont(f, 48);
12.   fill(0);
13.   text("Sample Text", 20, 100);
14.  
15.   PGraphics pg = createGraphics(400, 200, JAVA2D);
16.   pg.beginDraw();
17.   pg.textFont(f, 48);
18.   pg.fill(0);
19.   pg.text("Sample Text", 20, 100);
20.   pg.endDraw();
21.   image(pg, 400, 0);
22. }

Again, on Windows 7, the text on PGraphics is anti-aliased. There is no reason to have a different behavior, since the main drawing surface is a PGraphics like any other.