The answer appears to be: a lot, patent-wise.
You may recall around the end of last year it appeared that Apple was in a world-wide patent war against Google's mobile OS and its incarnation on various devices primarily produced by Motorola and Samsung. Apple didn't seem to be making much headway at first, failing to get injunctions and having some problems with counter-suits.
That may be about to change, at least in the US. A few days ago, Florian Mueller published a long piece on his FOSSPatents blog detailing Apple's latest salvo. Apple is asking for a preliminary injunction against Samsung's Galaxy Nexus, the flagship product in the most recent line of Android releases. Mueller covers the maneuverings in the US cases so far - Apple's first suit hasn't gotten them the injunctive relief they wanted and is on appeal, and most importantly Apple's new lawsuit is based at least in part on four patents that haven't been brought into the fray yet.
Mueller's opinion is that these patents, three of which issued only recently, are quite strong. I have read them over and I must say that I'm impressed. Even though they're 2011 issues they have filing dates of 2005 and 2004, and incorporate patents granted during those years. They also have a truly impressive array of both patent and non-patent prior art cited, some of which goes years back from the patent filing date. If the USPTO allowed these patents in the face of all the listed prior art it will be a real challenge to invalidate them. It will be interesting to see if Google & co try that approach. Of course they will argue that their products don't infringe the patents anyway, but that's also going to be a tough case to make, as Mueller details.
An alternative would be to try to code around them. For example, the '172 patent describes a fairly specific interface for providing word-by-word correction/suggestion as users type:
in a first area of the touch screen display, displaying a current character string being input by a user with the keyboard; in a second area of the touch screen display that is between the first area and the keyboard, displaying the current character string or a portion thereof and a suggested replacement character string for the current character string on opposite sides of the second area; replacing the current character string in the first area with the suggested replacement character string if the user activates a space bar key on the keyboard; replacing the current character string in the first area with the suggested replacement character string if the user performs a first gesture on the suggested replacement character string displayed in the second area; and keeping the current character string in the first area and adding a space if the user performs a second gesture in the second area on the current character string or the portion thereof displayed in the second area.
It seems like using a different method of laying out the screen, or a different keypress for auto-completion, might be a viable workaround.
However, the '604 patent appears to be extremely broad and covers a fundamental technique for finding, ranking, and displaying heuristic search results. Again, I'm no lawyer, but I don't immediately see a way of coding around this patent without losing core functionality not just for Google mobile but for Google's entire search business.
Given my recent note about how patent infringement should be enforced, it's interesting that Apple is aggressively pursuing injunctions. In effect, they're trying to knock out their biggest competitor, and likely no amount of monetary settlement would dissuade them from this plan.