yes I think it is a leopard thing too. That seems to be when people starting having issues with it.
It seems that EXT2FSX has some incompatibilities with the EXT2 flag DIR_INDEX.
To enable write permissions in MACOSX, first you have to unset it in linux:
tune2fs -O ^dir_index [partition]
Be aware though that EXT2FSX version 1.4D4 is unstable on the MACTEL.
Here, it crashed whenever you unmounted a read-write partition.
And that caused minor corruptions on the EXT2 filesystem.
EXT3 should work with no problems.
But I also heard that there is no support for Leopard.
btw did you try to use hfs+ with journal disabled to share data between linux and osx. I'm using this with my volume for music. I have r/w access from linux and osx. Amarok is working fine on that volume and neither does itunes...
And there is another way. For my windoze-peers I have a ntfs external drive. And with the ntfs3g installed in osx and linux it works quite fine, but on this filesystem you have to deal with the problem of fragmented data. If you have much writing-access, you will run into big trouble, unless you have a windoze to defragment the volume periodic...
iMac 27" running Mac OS X 10.6 (Snow Leopard) and Ubuntu 12.4 (Precise Pangolin)
iMac G4 running Mac OSX 10.04 (Tiger)
HP Pavillion running Ubuntu 8.04 (Hardy Heron)
Mac Expert, Linux User, Windows Virgin
I'm not very familiar with file systems, but have you considered UFS? There is native support for it in OS X and according to this post over at Linux Forums, it should be easy to add read/write support in Ubuntu.
The only question is journaling capabilities. I don't believe UFS uses journaling but some sort of metadata logging feature instead. You may want to check out this PDF from Sun, which gives some comparisons of ext3 and UFS.
I use an unjournaled, case-sensitive, hfsplus partition to share files in ubuntu and os x and I am not experiencing this filesystem check every boot like you mention. Could there be another reason it is triggering a filesystem check at every boot?