I had an NSLU2 running unslung as well.  It was a  great device, but is now quite dated.

Here's a brief detail of the hardware diferences:

                         NSLU2                        Pogoplug v2                        Notes
Processor
  Architecture        XScale    (Intel)            Sheeva (Marvell)                Both are based  on ARM
  Speed                133/266MHz                1Ghz

RAM                    32MB SDRAM                256MB (DDR)

Flash/NAND        8MB                            128MB

USB                    2x USB1.1                    4x USB 2.0

Ethernet            100Mbit                        1Gbit

Linux Kernel        2.4.xx                        2.6.22

Unofficial SW        PlugApps                    uNSLUng

There are of course other unofficial SW distributions for both, but these are the big ones. You 

Pogoplug also has some other nifty features over the NSLU2:

1. Abilitiy to transcode video files to a web-friendly version
2. Ability to access files remotely through the my.pogoplug.com website
3. Ability to access filesusing client software for the IPhone, Palm Pre, Blackberry,Windows, Linux

With the stock firmware no. There's rumor of them supporting UPNP in the future.

Or, if you're adventurous you can install PlugBox Linux from plugapps.com and then install and configure Samba.