Firstly, if you intend to use vanilla 2008, you won't be able to reliably use the APM module in the future due to inherent problems with WMI in 2008. Search the forums for "2008 lsass" for reports of the problems - Solarwinds doesn't recommend it.
As for 'clusters' (I presume you mean failover in the event of engine/poller loss) - Solarwinds support has previously said that they are intending at some point to release a 'failover engine' which might also accommodate switching from a broken 'live' database. They've only ever given me a very basic overview of what it will do.
Solarwinds does currently offer hot standby, polling and web server engines but I don't have any experience with them myself. However, it does look to me like they would all need access to the same database, which is a fairly big point of failure.