The main thing to tackle is booting up the xServes. Luckily, Intel xServes (which yours should be if they're running 10.6) have Lights-out-Management (LOM) that should support this sort of thing (all Macs support Wake-on-LAN, but that only works on a local network).
Apple Remote Desktop (ARD) has support for this. It looks like you'll need to forward a few ports (looks like 623 for the LOM, and then a bunch for ARD - do a search), unless you have some sort of VPN access to your home network.
The basic method will be, use ARD to power up your xServes, then remotely connect via ARD or a VNC client, build your Mathematica app and run it. Then shut down the xServes.
If you have a Mac with you, all this can be done via ARD, but if you want to do it with an iPad, you'll need your desktop Mac running whenever you want to remotely access it. The process there would be to enable Screen Sharing (in the Sharing panel of System Prefs), forward the appropriate ports, then use an app like Screens to connect to your desktop Mac, run ARD and Mathematica remotely on it, and do the above.
If you don't have your desktop Mac running all the time, you could in theory use Wake-on-LAN to turn it on, then access it with Screens, but that requires some other device to be always on, remotely accessible and capable of sending the WoL packet to the Mac (some routers are capable of this, typically with 3rd party firmware).
Best Answer
This isn't actually true. With ARD/Screen Sharing, you can leave the mac logged-out, authenticate when you connect with ARD, and then log in to your user account after connecting.
You just need to make sure the Mac isn't set to go to sleep when no one is logged in.
With TeamViewer, you would need to have the account logged in, but it should still be accessible if you have the account locked or using fast user switching. Those features allow an account to stay logged in, but prevent it from being used (physically or remotely) until authentication.