Well I was just told about rhomobile and was happy to find that they provide some tools to do this. I can't say I've tried them but I'm really impressed with the current set of mobile devices, particularly the iphone and android and see myself developing something from them in the near future. My preference being Android.
If I could write something for android but still export it to the iphone and symbian phones I would be very impressed. The only things on my mind are whether there are many restrictions in place if you want to write something that works on all devices. Since essentially some devices and their operating systems are more advanced than others.
Still this is worth knowing about and is exciting. Especially since it appears to be free.
edit 20090527 16:59
Just tried the on my means application written using this software and it's just terrible. Sadly the interface just didn't seem to work. I couldn't try the others out for various reasons so I can't give this a very favourable recview. Thought I'd google for other mobile frameworks and found there are a few out there. Haven't tried them yet but I hope they're better.