IMAP, or even POP3
If you have an "unlimited" data account (yeah, right...) just set your POP3 client to poll once a minute. T-Mobile will have to poll your account anyway, then the SMS transit time is non-zero, so you wouldn't see much delay compared to the "push" service.
The annoying thing is that there exists a public, unencumbered standard for mobile email - RFC4550 and chums. Essentially, it is IMAP IDLE plus a bunch of other tricks for cutting OTA bandwidth usage.
You aren't seeing it on handsets because:
1) it's hard.
2) it takes control of email away from the operator, so there's no incentive to spend loadsa cash making it work right (see 1 above).