I was also wondering whether this was just for BTC or ETH but it appears that the device can be used for several different coins/altcoins. I'll probably pick one up soon. There's no mention of STEEM anywhere on their site so I would imagine not. Here's what the Ledger website claims it is compatibile with :
Bitcoin
Ethereum
Ethereum Classic
Dogecoin
Litecoin
Zcash
Dash
Ripple
Stratis
Komodo
Some other Ethereum tokens can be managed using Ledger Nano S and Ledger Blue with MyEtherWallet only:
all the ERC20 tokens
1ST
ANT
ARC
BAT
BeerCoin
BCDN
BNT
CFI
CRB
CryptoCarbon
DAO
DAO_extraBalance
DGD
DGX 1.0
EDG
EMV
GNO
GNT
GUP
GT
HKG
HMQ
ICN
JET
LUN
MCAP
MCO
MGO
MDA
MIT
MKR
MLN
MNE
MYST
NxC
PAY
PTOY
PLU
QAU
QRL
REP
RLC
RLT
DICE
ROUND
SGT
SNGLS
SRC
SWT
TaaS
TIME
TKN
TRST
Unicorn
VSL
VERI
WINGS
XAUR
That is correct, ERC 20 Tokens are just linked to an ethereum address so any ERC 20 Token can be held on ledger wallet. Currently they don't have steem support, but they are leading the pack in hardware wallets and have the best support (IMO) for current and future coins which means that it is only a matter of time before they add Steem support. I forgot to mention but the ledger nano s can alsu be used as a 2 factor identification device much like google authenticator, which would let you lock up your steem if it is in a wallet that is compatible with 2FA.