First, you'll need to have already set up iCloud on your Mac. There are ways around this but that fills many threads/forums! Go to your user/Library/Application Support/iCloud/Accounts. You should see a couple of files with nothing but a (probably) 8 number 'name.' One is the ".lock" file which lets the OS know the settings have been properly created. But the point is, that number is your unique iCloud ID. Write it down or just leave the Finder window open to it.
OK, Calendars iss fairly easy:
1. On your iPhone (or other iOS4 hardware), open Settings->Mail, Contacts, Calendars.
2. Select "Add Account."
3. Select Other (since none of the included choices would work, anyway!).
4. Select "Add CalDAV Account.Now you'll need to enter a lot of stuff, including that iCloud ID number
1. In the "Server" area you need to type the following (remember to have your iCloud ID handy).
2. https://p03-caldav.icloud.com:443/IDnumber/principal
3. In "User Name" enter your iCloud username (probably your iCloud/MobileMe email address).
4. In "Password" enter your iCloud password (I bet you saw that coming, right?) 
5. You can enter anything you want in "Description." By default, it will probably look like what you typed in the "Server" area.
6. Press "Next"
7. You should see the 'gear' start to spin and an alert saying you've succeeded.
Contacts is basically the same as above.
1 Proceed with steps 1 through 3 above.
2. This time select "Add CardDAV Account."
3. If you've set up Contacts on your Mac, you can look in ~/Library/Application Support/Addressbook/Sources/ and see that there is probably one excessively larger HEX numbered folder. Go ahead and click the folder to see inside.
4. Open the Configuration.plist (using QuickLook is fine).
5. You'll see your iCloud ID in several places. The first one will serve us fine. You should see a url similar to the one you typed above. We don't need all of it. Just leave the window in whatever app you are looking at it open so you can type in the correct info in the iDevice.
6. Same routine as above, we need to enter a Server name, User Name, Password and a Description.1. The Server name is most of what you see in the .plist: https://p03-contacts.icloud.com:443/IDnumber/carddavhome
2. The rest of the fields use the same entries as above, except I'd suggest naming the address book something different than your Contacts. I used "iCloud Calendar and iCloud Contacts, restively. Of course, you don't have to be as imaginative. 
3. Press the "Next" button.
Don't be surprised if this takes a while to actually change anything in your Calendar or Contact list. Being a free service, I don't think Apple put as much effort into speed as they did into stability.
Note that you can probably get away without using the "p03-" part of the url. In fact, the enteries already have what will probably work! For the Address Book, you can likely just type: "cal.icloud.com" The 443 is the default Port. iCloud should be able to populate the rest of the server name by cross checking the info you enter for your Username and Password. For Contacts, just change "cal" to "contacts"
Be aware that populating your iPhone with data from iCloud may not be automatic the way it was with MobileMe. You may see delays each time you open Contacts or Calendar. And it may just not work without some further "encouragement!" Finally, don't be discouraged, there are plenty of people who claim nothing has worked for them. Much of this may be due to poor typing skills or poor reading of the instructions. In other words, "It worked for me so what do I care about you!?"
Seriously, if it doesn't work after two tries, let's try something else. At least that's what the skydiving instructor told me about the two lanyards on every parachute...
The above is simply my experiences and info from several sites:
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