Will SRV records ever become usefull?
Solution 1:
I think you'll only see it used for new protocols.
The MX records that SMTP uses are equivalent to SRV records with a fixed port and no weight. In other words, they've already adequately solved the same problem.
Since, for example, HTTP has to preserve backwards compatibility with the current mechanisms, anybody that started using SRV for HTTP would still have to provide an alternate mechanism anyways, and nobody wants to maintain two ways of doing the same thing. (say, some load balancers and some DNS SRV records...) And if sites aren't going to publish SRV records (because it just creates unnecessary work), there's no motivation for clients to support SRV records nobody is publishing.
Solution 2:
They're pretty useful in an Active Directory domain.
Solution 3:
SRV records are also in use by many Kerberos-enabled services and clients. This is especially true on a particular machine where /etc/krb5.conf (or it's equivalent) is either not readable or missing; a SRV record lookup will be performed to find the KDC.
Solution 4:
Apple's Bonjour technology (aka zeroconf) makes extensive use of this. If you haven't seen it in action -- check it out. It allows you to auto-discover printers, routers, bonjour-enabled webpages, etc.
There is a BSD-licensed Apache module called mod_bonjour which allows you to advertise websites via multicast DNS. You can also advertise sites via SRV records and regular DNS, but I believe that only Safari will be able to detect them.
The Zeroconf webpage has a pretty good explanation about how it works -- I'd recommend checking out the book too if you're interested in the technology.
One of the big problems with this in general is that DNS people don't consider themselves to be in the service discovery business, and paranoid security people consider the ability to discover services to be a security risk.
Solution 5:
Indeed, most applications don't yet support it.
Those that do are those where it's common for the domain in the target user's identity to be different from the hostname to which the client software must connect. Hence why it's used in SIP, and also in Jabber (XMPP).
SMTP doesn't need it because that always had MX records from the outset.
The benefit of being able to use a different port is relatively minor, so other protocols haven't started using it much.
There's an Internet Draft that proposes an http+srv: URI scheme, but note that there's no current proposals to add SRV lookups to standard http: URI requests.