No problem
There's an app called iWOL (£1.79) that allows you to send a magic packet over the internet or LAN to a specific machine. Can't say I've tried it, but don't see why it wouldn't work.
Again, if you're doing it from the WAN, you'll have to forward the port (port 7 UDP I think) from the WAN to the target machine. The downside with this is that you can only power up one machine on your LAN from outside said LAN. If this isn't a problem for you then this app will probably do what you want.
If you want the ability to power up multiple machines on your LAN from outside the LAN, this approach is useless I think.