Any time I get a new machine I run my non-scientific, hand-rolled Java benchmark app on it, and then publish the results. Here's the breakdown, including my new iBook:

Int     Double  String  IO      Total   Platform
-----   ------  ------  -----   ------  -----------------------------------------------------------

0.632   1.092   2.048   0.655   4.427   Sun Ultra 10/440 (128M RAM, Java 1.2.1)
0.104   0.726   3.507   0.516   4.853   Apple Macintosh G4 (dual 877mhz, 512M, Java 1.4.1)
0.549   1.425   2.753   0.67    5.397   Sun Ultra 5/400 (128M RAM, Java 1.3)
0.16    2.31    3.07    0.33    5.87    P700 Desktop, 256M, Java 1.3
0.266   3.547   3.719   0.515   8.047   Gateway E5200 (Dual PIII 450, 128 Megs RAM, Java 1.3)
1.102   2.86    3.296   1.15    8.408   Ultra 1 200E, 256M, Java 1.2.1
0.261   0.982   7.687   0.588   9.518   Apple iBook 900 (640M RAM, Java 1.4.1)
0.07    0.06    10.065  2.423   12.618  P600 Celeron Laptop, 256M, Java 1.2.2
4.825   19.903  1.439   0.89    27.057  HP 9000/800/N4000 2x440, 4G (under heavy load)
2.207   5.423   22.302  4.305   34.24   Sparc 10 (2x50mhz), 128M, Java 1.2
0.895   18.134  62.011  3.782   84.822  Sharp Zaurus SL-5500, Jeode VM

The list is sorted by total time. You might want to import this table into a spreadsheet to play with different sorts. From a quick glance, it looks like the iBook loses out in the String processing area for the most part, being half as fast as its big brother G4. My old Sun Ultra 10 is still king, at least in total time. Of course you have to take all these numbers with a grain of salt: my benchmark is highly unscientific, the conditions weren't controlled, and it realloy tests the JVM implementations on each platform as much as it tests the platforms actual speed. Still, fun to compare!