Same issue occurs in SeaMonkey and Fx - not 100% CPU, but easily 50%. Sample flash video:
http://www.cnn.com/video/#/video/crime/2009/05/20/seg.cb.langford.police.beating.cnn?iref=videosearch

$ top |grep 15186
15186 gg2xp     20   0  267m  88m  32m R 48.9  8.8   1:03.07 seamonkey-bin      <Flash playing
15186 gg2xp     20   0  267m  88m  32m R 46.8  8.8   1:04.48 seamonkey-bin      
15186 gg2xp     20   0  267m  88m  32m R 51.1  8.8   1:06.02 seamonkey-bin      
15186 gg2xp     20   0  267m  88m  32m R 56.5  8.8   1:07.73 seamonkey-bin      
15186 gg2xp     20   0  267m  88m  32m R 51.8  8.8   1:09.29 seamonkey-bin      
15186 gg2xp     20   0  267m  88m  32m R 52.8  8.8   1:10.88 seamonkey-bin      
15186 gg2xp     20   0  267m  88m  32m R 41.1  8.8   1:12.12 seamonkey-bin      
15186 gg2xp     20   0  267m  88m  32m R 26.2  8.8   1:12.91 seamonkey-bin      <Flash paused
15186 gg2xp     20   0  267m  88m  32m R 16.6  8.8   1:13.41 seamonkey-bin      
15186 gg2xp     20   0  267m  88m  32m R 13.3  8.8   1:13.81 seamonkey-bin      
15186 gg2xp     20   0  267m  88m  32m R 17.6  8.8   1:14.34 seamonkey-bin      
15186 gg2xp     20   0  267m  88m  32m R 17.6  8.8   1:14.87 seamonkey-bin      
15186 gg2xp     20   0  267m  88m  32m R 21.2  8.8   1:15.51 seamonkey-bin      
15186 gg2xp     20   0  234m  85m  31m S 35.2  8.5   1:16.57 seamonkey-bin      
15186 gg2xp     20   0  234m  85m  31m S  1.7  8.5   1:16.62 seamonkey-bin      <Flash killed
15186 gg2xp     20   0  234m  85m  31m S  3.0  8.5   1:16.71 seamonkey-bin

Build identifier: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.1b5pre) Gecko/20090520 SeaMonkey/2.0b1pre

Firefox:
$ top |grep firefox
15602 gg2xp     20   0  313m  78m  29m R 50.5  7.8   0:17.96 firefox            <Flash playing
15602 gg2xp     20   0  313m  78m  29m R 41.1  7.8   0:19.20 firefox            
15602 gg2xp     20   0  313m  78m  29m R 48.5  7.8   0:20.66 firefox            
15602 gg2xp     20   0  313m  78m  29m R 50.5  7.8   0:22.18 firefox            
15602 gg2xp     20   0  313m  78m  29m R 53.4  7.8   0:23.79 firefox            
15602 gg2xp     20   0  313m  78m  29m R 50.0  7.8   0:25.30 firefox            
15602 gg2xp     20   0  313m  78m  29m R 28.2  7.8   0:26.15 firefox            
15602 gg2xp     20   0  313m  78m  29m R 18.9  7.8   0:26.72 firefox            <Flash paused
15602 gg2xp     20   0  313m  78m  29m R 25.2  7.8   0:27.48 firefox            
15602 gg2xp     20   0  313m  78m  29m R 20.9  7.9   0:28.11 firefox            
15602 gg2xp     20   0  313m  78m  29m R 19.6  7.9   0:28.70 firefox            
15602 gg2xp     20   0  313m  78m  29m R 18.6  7.9   0:29.26 firefox            
15602 gg2xp     20   0  313m  78m  29m R 17.6  7.9   0:29.79 firefox            
15602 gg2xp     20   0  313m  78m  29m R 18.9  7.9   0:30.36 firefox            
15602 gg2xp     20   0  296m  75m  28m S 17.2  7.6   0:30.88 firefox            
15602 gg2xp     20   0  296m  75m  28m S  4.3  7.6   0:31.01 firefox            <Flash killed
15602 gg2xp     20   0  296m  75m  28m S  5.3  7.6   0:31.17 firefox

Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.0.10) Gecko/2009042523 Ubuntu/9.04 (jaunty) Firefox/3.0.10

$ apt-cache policy flashplugin-nonfree
flashplugin-nonfree:
  Installed: (none)
  Candidate: 10.0.22.87ubuntu2
  Version table:
     10.0.22.87ubuntu2 0
        500 http://archive.ubuntu.com jaunty/multiverse Packages
$ uname -a
Linux gg2xp 2.6.28-12-generic #43-Ubuntu SMP Fri May 1 19:27:06 UTC 2009 i686 GNU/Linux

nVidia Corporation NV25GL [Quadro4 900 XGL] (rev a3), Flash
10.0.22.87 (Note: same w/compiz effects on or off).