The short answer is yes. For more information, read on.
Because your URL is at findpresence.com, you might think that your data is transferred through our server. This is not the case unless you are using the EasyConnect service, in which case skip to the next section.
The only information we see is the location of your Mac on the internet. As soon as you see the login page, you're looking at your Mac, not our server.
In more technical terms, we store your latest IP address and external port number. Your Mac's name is translated into an HTTP redirect to your Mac using this information. After the redirect, some browsers will display the findpresence.com URL, while others will display an IP address. In all cases, your browser is talking directly to your Mac.
EasyConnect works by transferring your connection via our server, in a similar way to how Skype works. In the interests of full disclosure, you should know that it would theoretically be possible for us to eavesdrop on your data. Of course, there's absolutely no way we would do that, but you do need to know that you're trusting us by using this service. (You're also trusting us by installing our application on your Mac in the first place, as with any other application.)
But if you're concerned about this for some reason, your alternative is to configure your network so that you don't need EasyConnect.
Presence uses secure http, also known as https, to protect your data all the way from the web browser (or iPhone/iPad) you're using to your Mac at home. This is the standard method of providing encrypted web-based information over the internet. This means that, unlike with a normal web site, your data cannot be observed by anoyone else while it's in transit over the net.
https is the system used by online retailers and internet banking web sites. You should find this reassuring.
If you're interested in finding out more about https, Wikipedia is a good place to start.
Presence automatically creates a "self-signed certificate" as part of providing you with a secure web transfer. A self-signed certificate is one that has been signed by its own creator (the Presence application running on your Mac in this case) rather than by an independent certificate authority (a company such as VeriSign). This is why your browser displays the warning.
It is necessary for the certificate to be self-signed because it is generated automatically. To get a CA-signed certificate you would need to have your own domain at a fixed IP address, then to apply, pay for and install your own certificate. Clearly this is not practical for most Presence users.
In the future, Presence may allow users to provide their own certificate if there is demand for this.
No, not when it's done properly.
A firewall stops connections coming from the internet to your computers, and is generally built into your Airport, router or modem. Its default setting is to let nothing at all through; this is a easy and secure option to protect from unwanted intrusion, but it's not the only secure option.
When you use an application like Presence you allow it to open a "port" in your firewall. But don't think of this as a hole in a wall that allows you to get in and go anywhere - access through a port is much more controlled: it allows connection to one single application or service on one computer.
So, at this point the only security concern is whether that single application is secure enough to prevent unwanted intruders. As described above, the combination of https and Presence's password protection does indeed make it secure enough to be visible to the internet.
Security and networking can be complicated topics. If you're interested or concerned, I encourage you to read up on the subject, as the more you understand on this subject the better you'll feel about about using Presence and about your internet security in general.