Your estimated reward on Slush is based off live data and calculated using a very strong/sensitive anti pool-hopping algo, counting time since last share the highest.
The most significant/important shares are the ones your miner(s) delivered shortly before a round ends.
Depending on how many seconds ago your miner(s) delivered the last shares, the lower the current estimated rewards figure will be.
It's normal for that value to fluctuate quite a bit (together with other factors like total pool hashpower and your current luck while mining).
Generally, expect a relatively high variance (compared to other pools) when it comes to that anti pool-hopping algo, which in the long run will still yield a solid average and should be of little concern if you plan to mine there for a longer time.
Just remember all it takes for the estimated reward to drop harshly is to take your miner(s) offline for as little as a minute or less. Keep that in mind when planning any offline periods, only do that right after a round is finished.
On the other hand, jumping in late i.e. on a long running round isn't a bad idea. After mining some 20-30 Minutes into the end of even a 14 hour period, your estimated reward will be a full 100% (even if you didn't mine at all in the 13h30m earlier in the round). That's how extremely sensitive the anti pool-hopping algo is at slush.
(for comparison, leaving the slush pool during i.e. a 12 hour round for just 30 minutes can drop your reward for that round to 0, if it finishes before your miner(s) get back online and recover within the next minutes).
Basically all you can do is have your miners online 100% of the time, absolutely minimize downtimes and make sure your network connection is always up&stable.
Then you don't have to care much about estimated rewards for the current round.
Unless it's significantly below your typical average (indicating possible loss of hashpower i.e. due to a faulty/quit miner in your farm), it's of little importance, its fluctuations are completely normal.
PS.
If you have a very small/slow miner that takes longer times to deliver shares, the estimated rewards value will jump around wildly, only the unconfirmed reward earned immediately at the end of each round counts.
(I think there's a note on the account page addressing the effects of slow miners on the displayed data accuracy)
Only if you see absolutely nothing displayed permanently (like your miner isn't delivering any shares), then you should investigate your setup (correct data entered? correct address and pool, miner name and password 100% correct? miner using stratum protocol? etc.)
Remember some address/port on slush changed and this important change may not be included in your GUIminer default settings.
Current slush pool address
stratum.bitcoin.cz:333310.03.2013
Default mining URL for Stratum is stratum.bitcoin.cz:3333. If you're still using api.bitcoin.cz, please fix your URL to prevent fallback to deprecated Getwork protocol.
