Despite of all the sneering and eye-rolling, I got myself an iPad to replace my lab-notebook almost a year ago. I do not regret this decision.
For managing your references, you can use the iPad version of the famous OSX application "Papers". You can either run it only on the iPad or sync it with your installation of Papers on your desktop machine. Now it also allows annotations, but I think they are only visible in Papers on the iPad and Papers on OSX.
I annotate my papers with comments and highlighting in iAnnotate, which niftily saves the modifications in the PDF itself. So, if you send the document to a friend or share it for a journal club, non-iPad and non-Apple users can benefit from that too.
Personally I am also a fan of "iSSH", which is an SSH- and VNC-client. A lot of people say it is not possible to type reasonably fast on the on-screen iPad keyboard, but I cannot agree to that. Most other iPad users I met also confirm that they can type almost as fast on the on-screen keyboard as on a normal keyboard. If you really write a lot, you can always get the bluetooth keyboard that goes with the iPad. I use it when I know that I will be scripting a lot via SSH.
In my previous lab, we used Google Docs a lot. You can use the app "Office2 HD" for accessing your Google Docs, which works fairly well with documents and spreadsheets. It can also open Word and Excel documents, so for me it was a no-brainer to get this app too.
Several apps exist for scribbling down notes, making sketches or layout mind-maps.
Remember the Milk is releasing an iPad version for their service soon too -- in case you need to manage your TODO-lists, notes and deadlines online. Right now, you have to use the iPhone version on the iPad, which works well but does not make use of the full-screen of the iPad.
Apart from that, I also have some biology books on it. This comes in handy when you need to look something up that is not covered in sufficient detail on Wikipedia. You can use Apple's iBooks and their book store, or alternatively, you use the Kindle-app and buy books in Amazon's on-line store.
I would say the iPad is a valuable tool and definitely a good replacement for a lab-notebook.
UPDATE Here is a listing of 10 mobile apps (most of them on iPad) that are relevant in bioinformatics: http://www.fiercebiotech.com/slideshows/10-mobile-apps-life-sciences?img=0
2nd UPDATE The Apple Store features iProteome (http://itunes.apple.com/ca/app/iproteome/id524883731) now, which is described as "[...] a mobile application to find information, tryptic peptide sequences and backbone fragmentation ions commonly found in mass spectrometry." (As seen by @attilacsordas on Twitter).
iPad is a consumer product that can only display Apple approved entertainment - it is very unlikely that you could be productive with it
What kind of bioinformatic "things" do you want to do on (any) tablet?