Much like previous hits Peggle and Bejeweled, the PopCap team has produced a first-class iPhone port. On my 3GS it runs smoothly and the core gameplay is substantially unchanged from the desktop version, despite the smaller screen (you can get a free gameplay taste with the online version). Minor tweaks include placing your plant "catalog" along the left side of the screen and adding handy highlights which show you exactly into which row and column you're placing your plants.
Plants vs. Zombies is available now for iPhone and iPod touch for a bargain price of $2.99. Go get it; you won't be disappointed.