Can confirm this.
Updated from mjpegtools_1.9.0-0.0ubuntu2_i386.deb (which gave "Illegal instruction" on using mplex) to mjpegtools_1.9.0-0.0ubuntu3_i386.deb and now (at least) mplex works smoothly.
[Ubuntu Jaunty,
Athlon XP 1700+,
Linux 2.6.28-13-generic #45-Ubuntu SMP Tue Jun 30 19:49:51 UTC 2009]
Can confirm this. 1.9.0-0. 0ubuntu2_ i386.deb (which gave "Illegal instruction" on using mplex) to mjpegtools_ 1.9.0-0. 0ubuntu3_ i386.deb and now (at least) mplex works smoothly.
Updated from mjpegtools_
[Ubuntu Jaunty,
Athlon XP 1700+,
Linux 2.6.28-13-generic #45-Ubuntu SMP Tue Jun 30 19:49:51 UTC 2009]