If you are on iPhone/iTouch 2.2.1 - I'd suggest you do not upgrade to the latest version of the app (1.5.1) from the App Store - it crashes. There is a message with the update saying 'requires OS 3.0' - I didn't notice that. If you're ok with upgrading to OS 3.0 - fine - but I'm not - so I was stuck.
However on Mac OS there is a workaround - if you have the older Toodldo app in a backup. You need to put back the old 'Toodledo.ipa' file back into the correct location on your Mac, delete the app from the iPhone/iTouch, and then re-sync.
I'm not going to add more detail about how to do all this as this is an operation which could risk you losing functionality or data from your device if you don't know what you're doing.
I'm now backing to a working Toodldo app on 2.2.1 - but your mileage may vary.