Aye, we can hope that Axl will be on, but both the GNR forums and Axl's relatives seem to point to Axl having pulled out (a tweet from Axl's sister about watching Kimmel has now been deleted while the GNR forums has tweeted that Axl has had to cancel for unspecified reasons)
A small part of me - unlikely as it is - hopes Slash still appears. Kimmel recently (as in this week recently) followed both Axl and Slash on twitter at the same time so I can only imagine the two were due to appear together. Of course they could only announce one of them in advance as mentioning the other would give the game away re the reunion.
I really hope Slash (at least) does make a showing - simply because its getting to the point where they're beginning to try people's patience. Yes I know, this is GNR, the band that do this as a regular event, but a reunion has to be announced asap before people just get tired of all the hype surrounding it.