Apple has released Time Capsule and AirPort Base Station (802.11n) Firmware 7.4.1 that includes security fixes for the just-released models as well as the aforementioned older models.

Remote Access
The update also adds remote access to these models but does not add dual-band broadcast or the guest networking feature as these rely on hardware improvements.  Apple's support document details the following:

For AirPort Extreme (Early 2009) and Time Capsule (Early 2009) that ship with firmware 7.4, the firmware 7.4.1 update:

• Resolves an issue in which a client computer may be disconnected when waking from sleep
• Addresses an issue in which redirecting SMTP port services may disable IP-layer networking
• Improves the reliability of Back to My Mac-based disk sharing
• Includes recent AirPort security updates

For AirPort Extreme with 802.11n (Fast Ethernet), AirPort Extreme with 802.11n (Gigabit Ethernet), AirPort Express with 802.11n, and original Time Capsule models, the firmware 7.4.1 update:

• Enables remote administration and remote access to compatible USB connected drives via Back to My Mac (except on AirPort Express)
• Improves reliability when backing up to a Time Capsule via Time Machine
• Includes recent AirPort security updates

Airport Utility 5.4.1
Note that these firmware updates require that your first update to AirPort Utility 5.4.1.  You can obtain this latest Airport Utility update via Mac OS X Software Update.