Below is my response to a question on StackOverflow regarding using cross platform development tools when developing a mobile solution.
Key point here is, if you know going in you will have to develop on more than one device, then you owe it to yourself to do the due diligence and investigate all viable options… It just seems like common sense to me.
A little late, but my two cents…
I honestly believe you can very quickly prototype an application with Titanium Appcelerator and focus on the critical feature sets to determine if it is the appropriate tool for you.
All developers have there opinions and experience(s) which influence there comments; developers have different ways of learning and different levels of productivity… In the end, it comes down to how are you most productive with the tools available to you.
Since you are stating from the start that you want to deliver a solution an multiple platforms, I think it would be a poor decision on your part to not even spend a week or two investigating cross-platform frameworks and then making the decision based on your personal experience.
There is Titanium Appcelerator and there is also PhoneGap, where PhoneGap might help you is that there is the ability to extend/enhance the underlying framework through writing plugins (I wrote one for iphone ) and there is an android one on my blog also… this can fill missing gaps for you when you move across platforms.
Also since the UI in a phone gap solution is HTML5 Webkit based, it can give you a consistent look and feel across you devices if you like. Frameworks like jQTouch and JQuery Mobile are being used for UX with PhoneGap Application