If there's a relevant case going before the High Court this month, it's likely to be the government's appeal against the Court's ruling in the case of David Walker.
The previous court ruling did not state that IPPs are unlawful,
per se. However, it ruled that they way that are actually operated can, in some cases, breach the prisoner's human rights. David Walker (a sex offender) was given an IPP which, like all such sentences, meant that he couldn't be released from prison until it was considered safe for him to rejoin society. However, his lawyers successfully argued that the parole system prevented him from ever being able to prove that he could safely be released.
For details of the ruling, see here:
http://news.bbc.co.uk/1/hi/uk/6923434.stm
For analysis of the possible legal implications, if the government lose the appeal, see here:
http://news.bbc.co.uk/1/hi/uk/6924266.stm
However, as the last sentence in that report indicates, it would probably require another test case before the government would be required to make any changes. Even then, they'd only be required to change the way that IPPs operate; there would be no requirement for them to completely abolish them.
Chris