From HMXhenry on rockband.com:

Available on Xbox 360, Wii and PlayStation®3 system (September 20th, 2011):

-NEW RELEASES-

• Nirvana – “Heart-Shaped Box” X
• Nirvana – “Rape Me”
• Nirvana – “All Apologies”
• Nirvana – “You Know You’re Right”

The Nirvana new tracks will be available as “Celebrating Nirvana Pack”, and all tracks will be available for purchase individually on Xbox 360, PlayStation®3 system and Wii. Tracks marked with “X” will include Pro Guitar and Pro Bass expansions for $0.99 per song.

-PRO GUITAR/BASS UPGRADES-

These are upgrades to existing songs within the Rock Band Catalog

• Nirvana – “Smells Like Teen Spirit”
• Nirvana – “Breed”
• Nirvana – “Polly”
• Nirvana – “Territorial Pissings”
• Nirvana – “Lounge Act”
• Nirvana – “Stay Away”
• Nirvana – “On a Plain”
• Nirvana – “Something in the Way”
• Nirvana – “Lithium (live)”
• Nirvana – “Come As You Are (live)”

(These tracks and upgrades will be available in Europe on PlayStation®3 system September 21st)

Price:

$6.99, £3.49 UK, €5.29 EU (560 Microsoft Points, 700 Wii Points™) for “Celebrating Nirvana Pack”
$1.99 USD, £.99 UK, €1.49 EU (160 Microsoft Points) per song
$2.00 USD (200 Wii Points™) per song
$0.99 USD (100 Wii Points/80 Microsoft Points), £0.59 UK, €0.79 EU per song for eligible Pro Guitar/Pro Bass upgrade

Wow, that's a lot of upgrades! I'm surprised that Rape Me is coming out, but that's cool.

Not really a Nirvana fan, but I do appreciate hearing their songs once in a while. Rape me... wow. I'm sure they've censored that word in songs before, tho I will say that All Apologies is a nice pick for RB, pity that I won't ever play it due to no Pro upgrade. Also, disappointed about STILL no Frances Farmer Will Have Her Revenge On Seattle... 
 
  

Meh, Nirvana is not really my thing unfortunately. Congrats to all the fans though

Great now where's Paranoid Android.

@SMTDante89 said:

this!!!