There is an ipkg rsync which you could install. If you head over to http://www.openpogo.com/ there is a page describing how to set up ipkg.

UPDATE - Ok, reporting back on rsyncing directly from a Linux box to a Pogoplug modified with the "Openpogo" goodness mentioned on this forum.

I've installed the rsync package from the repo available at www.openpogo.com and all worked well for days.  I had an rsync process going literally for like 4 days straight (it was about 300+ GB of data that had to be synced for differences, I'm sure my ISP loves me now).

No problems whatsoever.... UNTIL.... I ran another rsync process tonight with the --delete option turned on.  Rsync on the Pogoplug pukes after about 20 seconds or so of deleting files/directories that don't exist on the source.  After rsync dies, the USB drive is unavailable.  If you try to CD to it while SSHed into the Pogoplug and do an "ls", it just hangs. Shows as mounted but it's not available.  Only way I've been able to recover is just reboot the Pogoplug with "reboot" command (I'm not local to the Pogoplug, so I can't just walk over and unplug it from the wall).

When it comes back up, all is well again...if I try rsync with --delete it dies pretty quickly again.  I'm guessing that this binary package of rsync just might not be completely compatible with either the kernel on the Pogoplug, or something else.  Probably just needs to be custom compiled for this specific device and kernel to work properly.

Here's my command line used to initiate rsync from source Linux box (edited to remove private host info and whatnot):

 rsync --delete --rsync-path /opt/bin/rsync --size-only --exclude '*RECYCLER*' --exclude '*System Volume Information*' -avze 'ssh -p 1212' /mnt/drobo/ root@hostname.com:/tmp/.cemnt/mnt_sda1/drobobackup/

When it dies on the source side, I get this error:

rsync: connection unexpectedly closed (8 bytes received so far) [sender]
rsync error: unexplained error (code 255) at io.c(463) [sender=2.6.8]

And here's what dmesg shows on the Pogoplug after a rsync crash:

Unable to handle kernel NULL pointer dereference at virtual address 00000040
pgd = c2c28000
[00000040] *pgd=07e11031, *pte=00000000, *ppte=00000000
Internal error: Oops: 17 [#1]
Modules linked in: xce tun ufsd(P) fuse hfsplus hfs vfat fat
CPU: 0    Tainted: P        (2.6.22.18 #18)
PC is at _ZN4UFSD14CFileMFTRecord14RemoveFileNameEPKNS_12NtfsDirEntryEPNS_10CIndexNameE+0x40/0x49c [ufsd]
LR is at __init_begin+0x3fff8000/0x30
pc : []    lr : [<00000000>]    psr: 80000013
sp : c32eddd8  ip : c7d356b8  fp : c32ede3c
r10: 00000000  r9 : c32ec000  r8 : 00000000
r7 : c4fd8300  r6 : c4fd8300  r5 : c6255f00  r4 : 00000000
r3 : 00000000  r2 : c60e78c0  r1 : 00000000  r0 : c4fd8300
Flags: Nzcv  IRQs on  FIQs on  Mode SVC_32  Segment user
Control: 0005317f  Table: 02c28000  DAC: 00000015
Process rsync (pid: 442, stack limit = 0xc32ec268)
Stack: (0xc32eddd8 to 0xc32ee000)
ddc0:                                                       bf042624 c01531c8
dde0: c32eddfc c32eddf0 bf0458b0 c60e78c0 c32ede1c c32ede00 bf058708 bf0458ac
de00: 00000000 c6255f00 00000000 00000000 c32ede2c 00000000 c6255f00 00000000
de20: c4fd8300 cf3ac180 c32ec000 00000000 c32edec4 c32ede40 bf058ed8 bf0690c8
de40: c32edea0 00000000 c32ede6c cf27a360 c32ede5a 00000008 00540000 00650068
de60: 00430020 00720061 00000073 00000000 c32ede9c c32ede80 bf043fd0 bf047e88
de80: 00000008 00000001 ce10e0b8 ce10e0c4 c32edebc c32edea0 bf0403c0 00000000
dea0: c083b8e0 c7d356b8 ce10e0b8 00000000 c0f7d448 c3e43bf8 c32eded4 c32edec8
dec0: bf043d4c bf058d08 c32edf04 c32eded8 bf041a40 bf043d2c c002b628 00000000
dee0: c32edf04 ce10e0b8 00000000 c3e43bf8 00000028 c002b628 c32edf24 c32edf08
df00: c015ee90 bf0419f4 ce10e0b8 00000000 cfdd5000 00000028 c32edf94 c32edf28
df20: c0161048 c015ee2c c5349758 c06258a0 00000000 00000008 cfdd5024 00000010
df40: 00000000 00000000 001fa000 001fa000 c0018380 001fa000 c00183b4 00000000
df60: c32edf7c c32edf70 c012a920 c0225bd8 c32edfa4 c32edf80 c01482a8 00000000
df80: 00000e68 00000004 c32edfa4 c32edf98 c01610d4 c0160fb0 00000000 c32edfa8
dfa0: c002b480 c01610cc 00000000 00000e68 bef63b00 80000000 00000000 00074c54
dfc0: 00000000 00000e68 00000004 00000028 00004000 000041ed bef63b00 00000004
dfe0: 0000002d bef63a58 00012944 400e5ccc 60000010 bef63b00 00000000 002e0000
Backtrace:
[] (_ZN4UFSD14CFileMFTRecord14RemoveFileNameEPKNS_12NtfsDirEntryEPNS_10CIndexNameE+0x0/0x49c [ufsd]) from [] (_ZN4UFSD8CDirNtfs6UnlinkENS_7STRTypeEPKvj+0x1e0/0x32c [ufsd])
[] (_ZN4UFSD8CDirNtfs6UnlinkENS_7STRTypeEPKvj+0x0/0x32c [ufsd]) from [] (UFSDAPI_Unlink+0x30/0x40 [ufsd])
 r7:c3e43bf8 r6:c0f7d448 r5:00000000 r4:ce10e0b8
[] (UFSDAPI_Unlink+0x0/0x40 [ufsd]) from [] (ufsd_unlink+0x5c/0x108 [ufsd])
[] (ufsd_unlink+0x0/0x108 [ufsd]) from [] (vfs_rmdir+0x74/0xbc)
 r8:c002b628 r7:00000028 r6:c3e43bf8 r5:00000000 r4:ce10e0b8
[] (vfs_rmdir+0x0/0xbc) from [] (do_rmdir+0xa8/0xec)
 r7:00000028 r6:cfdd5000 r5:00000000 r4:ce10e0b8
[] (do_rmdir+0x0/0xec) from [] (sys_rmdir+0x18/0x1c)
 r6:00000004 r5:00000e68 r4:00000000
[] (sys_rmdir+0x0/0x1c) from [] (ret_fast_syscall+0x0/0x2c)
Code: e50be034 e3530043 8a0000ba e1a0800e (e5d89040)